Role of residual stresses on interface toughness of thermally sprayed coatings

Thin Solid Films 415(2002)143–150

0040-6090/02/$-see front matter ?2002Elsevier Science B.V .All rights reserved.PII:S0040-6090?02.00488-1

Role of residual stresses on interface toughness of thermally sprayed

coatings

J.Lesage,D.Chicot*

Laboratoire de Mecanique de Lille,URA CNRS 1441,U.S.T.Lille,IUT A GMP ,Villeneuve d’Ascq,France

′Received 1August 2001;received in revised form 18April 2002;accepted 30May 2002

Abstract

In order to create and propagate a crack in the interface plane,Vickers indents were applied to polished cross-sections at the

interface of Cr C y NiCr coatings obtained by the high velocity oxy-fuel spraying https://www.wendangku.net/doc/ea18110419.html,ing this procedure,it is possible to 32calculate an apparent interface toughness.For as-sprayed specimen,it was shown first that the interface toughness varies like a function of one over the square root of the coating thickness.The value extrapolated to an infinite thickness is assumed to represent the adhesive properties of the coating.Same tests were performed on annealed specimen for which it was expected a severe modification of the residual stresses state.In these conditions,an apparent interface toughness was found independent on the coating thickness.Moreover,this value is equal to the value extrapolated for the as-sprayed specimen.This very important and new result allows to modify the model for the interface toughness in order to take into account a stress parameter.?2002Elsevier Science B.V .All rights reserved.

Keywords:Adhesion;Coatings;Hardness;Stress

1.Introduction

During the past decade,it was proposed to use the interfacial indentation test as an alternative to other tests in order to characterize the adhesive properties of ther-mal spray coatings w 1,2x .This test consists in measuring the length of cracks generated at the coating–substrate interface as a consequence of Vickers indentations.In order to interpret the results,various methods were proposed by authors searching to express either an interfacial toughness w 3x or a cracking energy w 4x .Since their models did not apply satisfactorily neither to our experimental results nor to others available in literature,we proposed in a previous work,a new representation that allows to define a critical load (the lowest one )necessary to initiate a crack w 5x .Using the results obtained on various substrate-coating couples and

*Corresponding author.Tel.:33-320-677326;fax:q 33-320-677321.

E-mail address:didier.chicot@univ-lille1.fr (D.Chicot ),jacky.lesage@univ-lille1.fr (J.Lesage ).hypotheses of the linear fracture mechanics we were able afterwards to propose a new relation to express an apparent interface toughness,taking into account the properties of both the substrate and the coating w 6,7x .A particular feature of thermally sprayed coatings is that they contain residual stresses after spraying and cooling w 1x .The residual stress state in coatings is very important since it can play a significant role on the service performance of mechanical parts w 8–10x .The goal of the present work is to examine if it is possible to found,or determine,a value for the interface toughness which would be independent on the coating thickness.The implicit idea is being to find an intrinsic parameter to represent adhesion of a coating on its substrate.

The paper will present first a short description of the interfacial indentation test and then its application to various coatings obtained by the high velocity oxy-fuel (HVOF )spraying process on different metallic sub-strates w 6x .Some results from Ref.w 4x for Stellite coatings obtained by the air plasma spraying (APS )process on stainless steel are also analyzed.

144J.Lesage,D.Chicot /Thin Solid Films 415(2002)

143–150

Fig.1.(a )Principle of the interface indentation test and (b )schematic

representation of the results.

2.The interfacial indentation test

The interfacial indentation test consists of measuring the length of the cracks generated at the coating–substrate interface,as a result of Vickers indentation tests performed at the interface using different applied loads.It does not require the use of glue like for tensile and shear tests but it requires a careful preparation of the coated sample.

Several models w 11–22x have been presented to deter-mine the toughness of brittle massive materials using indentation tests.Based on these previous methodolo-gies,a new procedure was applied to coated materials by emphasizing that the indented material which we will call the ‘interface material’,composed of substrate,coating and interface itself,can be considered as a unique brittle material w 6x .The interface indentation test methodology is the following:

1.obtaining a polished cross-section of the coated sam-ple,in order to perform Vickers indentation tests with different applied loads,for which the diagonal of the indent must be coincident with the coating–substrate interface;

2.for each indentation test,measuring the value of the half diagonal of the indent (d y 2)and the length of the crack (l ),both at the interface;

3.plotting these data as a function of the applied load in logarithmic scale,as represented schematically in Fig.1;relation between d y 2and P is linear and is called the ‘apparent hardness’line in the figure (see details in Ref.w 6x );

4.determining the coordinates of the critical point indi-cated in Fig.1(P ,a ),underneath which no C C interface cracking is observed upon indentation and which are used to derive the apparent interface tough-ness of the coating–substrate couple.During indentation,a plastic deformation zone is created by sharing the combined local properties of the coating and the substrate.Eventually,a local crack may be likely to occur in the interface plane if the fracture toughness of this composite interface material is attained.The purpose of the interface indentation test is to give a quantitative measure of the apparent fracture toughness.Some general formulations found in literature involve a ratio between elastic modulus and hardness of the material.In the special case of the interface material,we proposed earlier w 6x to express it as the square root of the ratio of the elastic modulus (E )divided by the Vickers hardness (H )at the interface,by considering the mean geometrical features of the substrate and coating couple:

1y 21y 2

E y H E y H ?.?.S C 1y 2

E y H s q (1)

?.1y 21y 2i

1q H y H 1q H y H ?.?.

S C C S where the subscripts i,S and C stand for interface,substrate and coating,respectively.

The mathematical reasoning and its statement were described elsewhere w 6,7x .By appropriately combining the critical parameters for cracking,P and a ,as C C defined in Fig.1,with the interface mechanical proper-ties,the apparent interface fracture toughness (Kca )may be defined as w 7x :1y 2

B E P E

C C F

Kca s 0.015(2)

3y 2a H D G C i

in a similar way than those proposed earlier for bulk materials by Lawn et al.w 17x or Anstis et al.w 18x for example.Although this type of relation is only valid for cracks that are longer than the radius of the plastic zone,it was emphasized that the crack line can be extrapolated at the critical point.In this way,P and a ,the C C coordinates of the critical point,can be used for the calculation of an apparent interface toughness Kca.The reason why we choose the critical point for the calcu-lation found its justification in the following idea.When a crack is formed,its propagation is aided by the flexion of the coating due to the action of the indenter which plays the role of a wedge inserted at the interface between the coating and the substrate.As an evidence,the propagation will depend on the coating thickness

145

J.Lesage,D.Chicot /Thin Solid Films 415(2002)143–150

Table

1

Mechanical characteristics of materials and calculated interface parameter Substrates Coatings H S

E S

H C

E C

1y 2

E y H ?.i

(GPa )(GPa )(GPa )(GPa )(1)Low carbon steel Cr C –NiCr 32 1.302109.0712510.24(2)Grey cast iron

Cr C –NiCr 32 2.141059.07125 5.93(3)Globular graphite cast iron Cr C –NiCr 32 2.561059.07125 5.47(4)Austenitic steel Cr C –NiCr 32 1.992109.071258.18(5)Low alloyed steel Cr C –NiCr 32 3.682109.07125 6.06(6)

Stainless steel w 4x

Stellite

1.65

203

3.73

175

9.40

Fig.2.Bi-logarithmic representation of crack length (a )vs indentation load (P ).As-sprayed specimen from the Cr C –NiCr deposits on (a )low 32carbon steel;(b )grey cast iron;(c )globular graphite cast iron;(d )austenitic steel;(e )low alloyed steel and (f )Stellite on stainless steel (w 4x ).

since a thicker coating will resist the flexion much better than a thinner.As a consequence,the slope of the crack line will be affected by the elastic behavior of the coating.The only point of the cracking line which is not concerned by this phenomenon is the crack initiation point P .Moreover,we will see there after that a unique C critical point can be obtained independently of the slopes of the crack lines when a suitable annealing treatment is applied to coatings of different thickness.In this case,it is the only point which belongs to all the lines.

This methodology will be applied now to as-sprayed specimen for which it is emphasized a significant role of the residual stresses on the apparent interface tough-ness.In effect,depending on coating and substrate,it is known that the sign,the intensity and repartition of residual stresses in the coating could be very different.For example,Kesler et al.w 8x found an increase of the residual stresses for lower thickness of a molybdenum coating contrary to Perrin et al.w 23x ,cited by Khor and Gu w 24x who found larger residual stresses for thicker coatings of a functionally graded material.

3.Materials and experiments

Samples were obtained from specimen of five various metals and alloys (low carbon steel,grey cast iron,globular graphite cast iron,austenitic steel and a low alloyed steel )coated with a chromium carbide (75%),nickel–chromium (25%)material using the HVOF spraying process.Rectangular specimen (120=15=10mm )were first sand blasted in order to eliminate 3impurities and to confer to the surface a mean roughness of 6–10m m favorable to the mechanical anchorage of the molten particles.Spraying was performed on the two largest faces of the samples.Thickness of the coating was varied between 200to 600m m.SiC paper from grade 80–1200was used for pre-polishing which was followed by a diamond final polishing.

At least two different coating thickness were tested for each substrate in order to observe the effect of the residual stresses on the adhesion.

Table 1collects the data and measurements concern-ing the materials tested:Y oung’s modulus (E ,literature ),

146J.Lesage,D.Chicot /Thin Solid Films 415(2002)143–150

Table

2

Coordinates of the critical point and apparent interface toughness for each substrate and thickness Substrates

Low carbon steel

Grey cast iron

Globular graphite cast iron

Austenitic steel

Low alloyed steel

Stainless steel w 4x

e (mm )0.270.300.400.470.370.580.270.560.360.440.24

0.360.560.220.290.380.49P (N )C 15.122.640.351.3 3.5 5.81 6.2 3.410.20.10.6227.712.216.7a (m m )C 52638495212711262137371320405058Kca (MPa m )1—

2 6.2 6.98.08.5 3.2 3.7 2.2 3.8 4.

3 5.6 1.7 2.9 3.9 3.2 4.3 4.9 5.3

Table 3

Coordinates of the critical point and apparent interface toughness for each substrate (annealed samples )Substrates Low carbon Grey cast Globular graphite Austenitic Low alloyed Stainless steel iron cast iron steel steel steel w 4x P (N )C 78.3710.748.2 4.727.1a (mm )Co 0.1180.0300.0340.0790.0200.074Kca (MPa m )

1—2O 9.4

3.8

4.4

8.4

4.8

6.0

Fig.3.Influence of coating thickness (e )on the apparent interfacial toughness (Kca )for the Cr C –NiCr coating deposited on (a )low carbon 32steel;(b )grey cast iron;(c )globular graphite cast iron;(d )austenitic steel;(e )low alloyed steel and (f )Stellite (w 4x ).Vickers hardness (H )measured with a microindentor

LECO under an applied load of 10N and the calculated

parameter which appears in the apparent inter-1y 2

E y H ?.i facial toughness formulae (Eq.(1)).

Five indentations were performed at each level of load in order to determine a reliable mean crack length.It is clear that the standard error on the mean crack length for a given load may be quite important (20%at maximum ).Anyway,enough data were obtained (sev-eral loads )to determine the value of P with sufficient C accuracy.

Fig.2represents the mean crack length (a )as a function of load (P )according to the film thickness (e )and for each substrate tested.Results obtained by Chou-lier w 4x on a Stellite coating are also presented in Fig.2using the same methodology.

As mentioned previously,the use of logarithmic coor-dinates allows to obtain a straight line for the crack length–load relation.Applying the linear regression method to the experimental crack points (a ,P )as well as to the apparent hardness points (d y 2,P )it was possible to determine the coordinates of the critical point (P ,a )and to calculate the apparent interface tough-C C ness Kca using Eq.(2).The corresponding values are given in Table 2.

Considering the results,it is seen that the critical load strongly depends on the thickness of the coating.For example it is 51.3N for a thickness of 470m m instead

147

J.Lesage,D.Chicot /Thin Solid Films 415(2002)

143–150

Fig.4.Bi-logarithmic representation of crack length (a )vs indentation load (P ).Annealed specimen from the Cr C –NiCr deposits on (a )low 32carbon steel;(b )grey cast iron;(c )globular graphite cast iron;(d )austenitic steel;(e )low alloyed steel and (f )Stellite on stainless steel (w 4x ).

https://www.wendangku.net/doc/ea18110419.html,parison between toughness extrapolated from Fig.3(as-sprayed specimen )and calculated toughness (annealed specimen ).

of 15.1N for a thickness of 270m m for the chromium carbide coating on low carbon steel.

Since the critical load P corresponds to the limit for C which no crack is formed during indentation,it is clear that differences in P values cannot come from the C bending of the coating.If it is assumed that no signifi-cant modification of the metallurgical bond between the substrate and the coating is induced by a change in thickness,then we have to consider that only the residual stresses are responsible of the variation of P with C coating thickness.

It is very difficult to measure the residual stresses in the bilk of a thermal spray coating and in particular in the vicinity of the interface.Anyway,if a parameter related to the stresses could be found (p (s )),it will be

possible to evaluate their effect.By using a factor 1y e ,2where e is the coating thickness,we obtained a linear variation with the calculated Kca (Fig.3).This result is very interesting because it can be extrapolated for an infinite thickness.This extrapolated value,Kca ,can be O considered as independent on the coating thickness and thus,the mathematical relations associated to this rep-resentation have the following form:?.p s Kca s Kca q

(3)

O 2

e What means exactly Kca and the second term o

f the O relation are the questions we will discuss in the following?

4.Discussion of the model

The stresses in the coating come from different sources:quenching of the impinging splat,restricted contraction,thermal mismatch between the coating and the substrate for the main ones.We will consider now two different levels of the remaining stresses after cooling.A level 1independent on the thickness,intrin-sically connected to the adhesive properties of the materials and to the process,and another level 2depend-ent on the coating thickness.

It is possible to reduce drastically the residual stresses by an appropriate annealing treatment of the samples w 8x .We will consider now the hypothesis that Kca O corresponds to a mechanical state independent of the level 2stresses.If this assumption is correct,the inter-facial indentation tests on annealed specimen should give a unique value for Kca (i.e.Kca ).

O

148J.Lesage,D.Chicot /Thin Solid Films 415(2002)143–150

Table 4

Influence of coating thickness on the residual stresses determined by X-ray diffraction

NiCr on low carbon steel normalized

NiCr on low carbon steel quenched e (mm )

0.240.450.560.220.410.52s (MPa )(annealed )w y 20y 35y 25y 15y 10y 30s (MPa )(as sprayed )

w y 195

y 100

y 60

y 190

y 130

y 40

NiCr coatings on low carbon steel normalized or quenched.

Fig.6.Influence of thickness on Kca,as applied to NiCr films deposited on low carbon steel substrate normalized and quenched.

Here,the annealing treatment was chosen to be 2h at 6008C and applied to the same chromium carbide samples.Interfacial measurements were then performed using the methodology described here above.

Table 3collects the results of the indentation tests and Fig.4gives their representation.It is remarkable to see that,for a given substrate,all the lines (a ,P )intersect at a same point of the apparent hardness line.That means that each coating and substrate couple admits a unique critical load and then,a unique value for Kca.Moreover,these values for Kca (including Stellite coating )agree very well to the extrapolated values obtained for the as sprayed samples (Fig.5).Examination of the results show that the annealing treatment have removed at least the part of the residual stresses dependent on the thickness (level 2stresses ).We do not know,if they exist,what are the intensity and the repartition of the level 1residual stresses.But we may imagine that they are involved in the global intrinsic process of adhesion of the coating on its substrate.Then they are implicitly taken into account in the Kca term which represents the adhesive properties O of the coating on its substrate.We will see now how to express the effect of the level 2residual stress in the model.

From works devoted to the study of the role of residual stresses on toughness of brittle materials,some relations were proposed in literature w 25x .These relations have a similar form than that of Lascar w 26,27x .In this

relation,toughness is expressed as a sum of a constant term Kc and of a term related to the stresses (s )O R surrounding the indent:2

1y 2Kc s Kc q

s a (4)

O R y p

where a represents the actual length of the crack and 2y 6p a coefficient associated to the geometry of the Vickers indent.

We will examine now if a similar form for Kca could represent the results obtained for coated materials.A possible form is,for example:2

1y 2

Kca s Kca q

s a (5)

O R Co y p

In order to apply this model,a is calculated from

Co the system:

1y 2

B E P E Co

C F

?.Kca s 0.015extrapolated value O 3y 2a H D G Co i ?.

ln a s a q 1y 2ln P apparent hardness line Co Co

The s of Eq.(5)can be calculated without any R knowledge of how could be the intensity and repartition of the stress.Further work has to be done in order to precise if it corresponds to the average stress at the interface,the difference between the stresses situated on

149

J.Lesage,D.Chicot /Thin Solid Films 415(2002)

143–https://www.wendangku.net/doc/ea18110419.html,parison between the s values calculated with Eq.(4)and residual stresses measured by X-ray diffraction,as applied to NiCr films R deposited on low carbon steel substrate normalized and quenched.

both sides of the interface or the mean stress in the coating.

This methodology will be applied now to APS NiCr coatings for which residual stresses were measured by X-ray diffraction at the surface of as sprayed and annealed coatings.In this particular case,the residual stresses are lower and tend to be null after annealing,at least at the surface of the coatings (Table 4).This result supports well our assumption that the level 2stresses can be removed by an appropriate annealing treatment.Fig.6shows that the relation between Kca and the thickness is similar to that obtained for the HVOF coatings.According to the proposed interpretation,it is possible to determine Kca and,using Eq.(5),to O calculate the residual stresses for each thickness.

A comparison between calculated and measured val-ues of residual stresses is found in Fig.7.The very good agreement between the values is very surprising and gives interesting prospects for further works devoted to the relation between residual stresses at the surface and Kca.5.Conclusion

Using experimental work performed on chromium carbide coatings obtained by the HVOF process,we showed the validity of an apparent interface toughness formulation to represent adhesion of coatings.Acknowl-edging the fact that toughness varies with the thickness of the coating,it was emphasized here that two levels of residual stresses have to be considered.A level 1,independent on thickness,corresponding to stresses involved in the intrinsic process of adhesion,and a level 2connected only to the thickness of the coating.Following this hypothesis we proposed to separate Kca in a term Kca independent on the thickness and a term

O associated to level 2residual stresses.Interfacial inden-tation tests and X-ray diffraction measurements per-formed on APS nickel chromium coatings before and after an annealing treatment support well the present argumentation.

Further work on the subject should deal with the precise determination or prediction of the two levels of residual stresses in thermal sprayed coatings.Acknowledgments

This work is part of a study supported by CAPES y COFECUBtype II project no.206y 97.References

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中国周边国际形势分析 摘要：我国疆域辽阔，东临太平洋，西接亚洲腹地，四周分别与东北亚、东南亚、南亚、中亚相邻。我国周边各地域的政治格局表现出横向的差异性及纵向的变动性。从东、南、西、北各地缘方向看，周边环境也呈现出不同态势，大致可归纳为“北稳、南和、东紧、西动”。从海陆地缘方向看，周边环境呈现出较大的差异性，大体可概括为“陆稳海动、陆缓海紧”。在这样的环境下，只有处理好周边关系才能实现中国的伟大复兴。 关键词：周边关系冲突矛盾共同发展战略安全 中国是当今世界上邻国最多的国家之一，维护中国的利益，处理好与周边国家的外交关系，消除危及国家安全隐患，加强对战略安全的研究，为中国和平崛起，成为世界强国创造条件。我国周边关系发展可具体分析为以下八个方面： 一、朝鲜半岛 进入21世纪后，美国政府对朝鲜采取强硬姿态，最终导致朝美第二次核危机的爆发，半岛局势再度趋紧。朝鲜半岛问题是东亚地区最大的冷战遗产，朝鲜半岛具有十分重要的战略意义，是各大国利益的交汇点，半岛两国的战略选择，将与大国因素相互作用，相互影响。在新时期初期的半岛局势仍将呈现出复杂多变的发展态势。朝鲜半岛是中国东北部安全的战略缓冲，半岛局势的紧张将破坏本地区的和平与稳定，也将影响中国现代化建设的进程，半岛南北双方真正走向和解，只有在中美等大国的支持下，才能取得实质性的成果。 二、日本 2010年是中日邦交正常化38周年。日本作为世界第二大经济强国，并且目前正处于转型过程，其走向将直接牵涉到我国的东部安全。日本政府对周边国家尤其是中国的强硬态度，严重影响地区的稳定，在历史问题上的错误态度也引起众多亚洲国家的不满，钓鱼岛问题也激化了中日两国在东海问题上的矛盾。虽然我国政府从大局出发，采取了理性和负责任的态度，但伴随着我国经济的发展和国际地位的不断提升，中日关系重新定位的过程中，矛盾和摩擦不可避免。 三、美国 在我国的周边政治格局中，美国是最具影响力的大国因素，也是对

一般过去式 时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991

中国与周边国家关系及未来发展趋势分析 （） 改革开放以来，中国的经济力量急剧增长，十多年来经济不断高速增长，国民生产总值和经济总量一再成倍加大，对外贸易的地域和规模也迅猛扩展。在2009年发生了世界性的经济危机后，中国的经济总量仍然保持了9%左右的增长。并且，中国的经济总量于2010年成功超越日本，成为世界第二大经济体。中国的崛起，引起了世界的密切关注，也引来了诸如“中国威胁论”等影响中国与世界关系的言论。 从大的格局来说，冷战结束，终结了中国与邻国间的政治分割。各国之间都基于各自长远战略利益与现实实际利益加以考、处理国家之间的关系。中国的邻国包括了诸多国情差异巨大的国家,在整个世界上是较为少见的，这就在一定程度上增加了中国与周边国家关系的复杂性。 在与我国相邻或隔海相望的东亚国家中，最为复杂的当属中日关系，其次为中朝与中韩关系。中国与日本关系之间，既有每每引起矛盾和冲突的历史遗留问题，又有紧密合作与竞争并存的现实关系。钓鱼岛、参拜晋国神社、历史教科书、慰安妇、日军遗留毒气弹、南京大屠杀等等的历史遗留问题。近几年，日本经济持续萎靡不振，而我们在今年年内有望超过日本，成为世界第二大经济体。这引起了日本国内右翼分子的不满与敌视，也使日本政府对我国的态度更加捉摸不定。同时伴随着日本在对外政策上加快谋求政治大国或“正常国家”的步伐，强化日美同盟，防范和牵制我国的行动不断升级，使中日关系在发展中又存在着许多隐患。但同时，我们也应看到，中日之间关系的友好一面。现如今，中国为日本的第二大贸易伙伴国和商品出口国，索尼、东芝、丰田、本田、马自达，在我们的大街上随处可见，我们的产品也大量销往日本。这都可以说明，我们同日本之间也有着不可分割的密切关系，这要求我们对待中日关系是要理智、客观、冷静。 我国和朝鲜、韩国之间的关系，虽不如与日本那样复杂，但也是扑朔迷离、纷繁复杂。在上世纪50年代，我们抗美援朝，与朝鲜军民并肩作战，将“联合国军”赶出了三八线，保卫了自己和朝鲜人民的家园，挫败了美帝国主义的阴谋。可以说，我们与朝鲜本应有着革命战友般的友谊，但是朝鲜核问题也使我们与朝鲜兄弟之间有些不愉快的小矛盾。我国与韩国之间的关系也是复杂多变。历史上，虽然我们帮助朝鲜军民抵御了侵略，但在韩国人眼中，我们却是造成朝鲜半岛分裂现状的凶手之一。两国关系在历史上长期处于敌对状态。但改革开放后，现如今，我国境内现有超过一百万的韩国公民居住，成为了世界上最大的韩国人海外居住地。我国的白菜、葱、蒜等农产品大量销往韩国，我国大量持有着韩国国债，持有量位居前三。同时韩国也向我国大量出口汽车等工业产品，两国经贸往来日益密切。同日本一样，我们与韩国也日益成为联系密切的利益共同体。 近年来，中俄关系的发展取得长足进步，两国高层互动频繁，涉及政治、军事、经济、科技、文化等诸多领域。横跨要大陆的俄罗斯，从沙俄时代到至今，一直是中国北方最大的邻国和影响中国国家安全最重要的因素之一。从地缘政治上，中俄之间有漫长的边界线，我国北方边缘方向仅有俄罗斯和蒙古两个国家，俄远东地区和蒙古的形势相对稳定，不存在重大现实热点问题和安全隐患。同时，我国和俄罗斯的睦邻友好关系处于良性状态，并且不会在短期内长生重大动摇。

中国周边形势分析 《香港商报》12月25日报道，朝鲜半岛局势紧绷之际，美国决定再加派核动力航母“里根”号开赴东亚，亚太海域将出现三个美海军航母战斗群。中国海军少将张召忠向媒体表示，当三个航母战斗群出现在同一地区时，标志着战争就要爆发。现在“里根”号、“华盛顿”号与“卡尔·文森”号三个航母战斗群会师，被舆论视为60年来对中国和朝鲜的最大军事挑衅行为。 自从南韩天安舰沉没、中日钓鱼岛之争和朝鲜韩国炮战，东北亚局势每况愈下，美国在东北亚的军事演习愈来愈频繁，规模愈来愈大。今天以三艘航母战斗群配置东北亚，比一九九六年台海危机的规模还要大。 众所周知，美国派一艘航母是战略威慑，两艘航母是备战，三艘以上则是作战。以今次规模来看，美国显然已开始备战，向世界发布东北亚必有一战的战争暗示。而如今东北亚各国都展开了大规模军事行动，其实，无论美国及其盟友打着什么旗号，最终目标都是中国，因为自从改革开放以来，特别是最近一段时间里，中国的崛起之势势不可挡，无论是在经济上，还是在军事上，中国的各个方面都时刻发生着翻天覆地的变化! 面对新兴大国中国的快速崛起，作为当今世界利益的既得者——美国及其盟友，会允许其他国家同他们分享猎物吗?纵观世界近五百年历史我们可以得出一个结论：一个新兴大国的崛起，也就标着另一个大国的衰落，没落的帝国必定会想尽一切办法阻止其衰落时间，尽一切力量打压和遏制新兴大国崛起的速度，这是一个历史规律，也是一个国家的本质，任何国家做任何事情都是把本国的利益放在首位。 2010年即将过去，我们回首今年中国周边发生的一切事情，其实都暗藏着种种阴谋，但归根结底都是为了对付中国，在中国周边无时无刻都存在一群野狼，他们时刻盯紧中国，恨不得把中国一口吞掉! 斯里兰卡每日镜报24日发表分析文章指出，日本、澳大利亚和新西兰等国家近期纷纷在其国防白皮书中将中国列为威胁或忧虑对象，一个美国领导的围堵中国的军事同盟正在形成。如果中国与这些国家未来发

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高中英语~词性~句子成分~语法构成 第一章节：英语句子中的词性 1.名词：n. 名词是指事物的名称，在句子中主要作主语.宾语.表语.同位语。 2.形容词;adj. 形容词是指对名词进行修饰~限定~描述~的成份，主要作定语.表语.。形容词在汉语中是（的）.其标志是: ous. Al .ful .ive。. 3.动词：vt. 动词是指主语发出的一个动作，一般用来作谓语。 4.副词：adv. 副词是指表示动作发生的地点. 时间. 条件. 方式. 原因. 目的. 结果.伴随让步. 一般用来修饰动词. 形容词。副词在汉语中是（地）.其标志是：ly。 5.代词：pron. 代词是指用来代替名词的词，名词所能担任的作用，代词也同样.代词主要用来作主语. 宾语. 表语. 同位语。 6.介词：prep.介词是指表示动词和名次关系的词，例如：in on at of about with for to。其特征：

介词后的动词要用—ing形式。介词加代词时，代词要用宾格。例如：give up her（him）这种形式是正确的，而give up she（he）这种形式是错误的。 7.冠词：冠词是指修饰名词，表名词泛指或特指。冠词有a an the 。 8.叹词：叹词表示一种语气。例如：OH. Ya 等 9.连词：连词是指连接两个并列的成分，这两个并列的成分可以是两个词也可以是两个句子。例如：and but or so 。 10.数词：数词是指表示数量关系词，一般分为基数词和序数词 第二章节：英语句子成分 主语：动作的发出者，一般放在动词前或句首。由名词. 代词. 数词. 不定时. 动名词. 或从句充当。 谓语：指主语发出来的动作，只能由动词充当，一般紧跟在主语后面。 宾语：指动作的承受着，一般由代词. 名词. 数词. 不定时. 动名词. 或从句充当. 介词后面的成分也叫介词宾语。 定语：只对名词起限定修饰的成分，一般由形容

M A: Has the case been closed yet? B: No, the magistrate still needs to decide the outcome. magistrate n.地方行政官，地方法官，治安官 A: I am unable to read the small print in the book. B: It seems you need to magnify it. magnify vt.１．放大，扩大；２．夸大，夸张 A: That was a terrible storm. B: Indeed, but it is too early to determine the magnitude of the damage. magnitude n.１．重要性，重大；２．巨大，广大 A: A young fair maiden like you shouldn’t be single. B: That is because I am a young fair independent maiden. maiden n.少女，年轻姑娘，未婚女子 a.首次的，初次的 A: You look majestic sitting on that high chair. B: Yes, I am pretending to be the king! majestic a.雄伟的，壮丽的，庄严的，高贵的 A: Please cook me dinner now. B: Yes, your majesty, I’m at your service. majesty n.１．［Ｍ－］陛下（对帝王，王后的尊称）；２．雄伟，壮丽，庄严 A: Doctor, I traveled to Africa and I think I caught malaria. B: Did you take any medicine as a precaution? malaria n.疟疾 A: I hate you! B: Why are you so full of malice? malice n.恶意，怨恨 A: I’m afraid that the test results have come back and your lump is malignant. B: That means it’s serious, doesn’t it, doctor? malignant a.１．恶性的，致命的；２．恶意的，恶毒的 A: I’m going shopping in the mall this afternoon, want to join me? B: No, thanks, I have plans already. mall n.（由许多商店组成的）购物中心 A: That child looks very unhealthy. B: Yes, he does not have enough to eat. He is suffering from malnutrition.

中国周边安全形势分析论文 当前，中国周边安全环境新变数与新“乱子”有所增加，不稳定与不确定性有所上升，在中国与周边国家的关系中，一些争议和摩擦连续出现，如中日钓鱼岛之争、中印边界领土争端、中菲黄岩岛对峙事件等，分析其原因不难发现： 一是因为中国周边地理环境复杂。中国拥有960万平方公里的陆地疆土，有2.2万公里长的陆地边界线，与中国接壤的国家有14个。中国还有1.8万公里长的大陆海岸线，与6个海上邻国的领海相接或相重叠。此外还有非接壤但有着密切关系的近邻国家等，因此，中国被称为“世界上邻国数目最多的国家”。中国不仅邻国数量多，而且这些国家的情况十分复杂多样，这在世界上较为少见。中国所处周边环境特别复杂，决定了在中国与邻国之间发生摩擦的概率要更大一些。 二是因为在中国与近邻国家之间，还存在着一些历史遗留下来的，未解决的边界问题。其中包括中印边界问题、中日钓鱼岛问题，还有南中国海问题。在这些领土和领海的划界问题上，虽然中国希望通过和平谈判的方式加以解决，但一些邻国不断采取各种手段，试图强化对争议地区的实际占领。这反映了周边某些国家的一种机会主义的心态，它们试图利用中国的和平主义政策，造成对中国不利的既成事实，谋取现实的利益，或者强化自身在未来边界谈判中的地位。 三是由于中国崛起带来的冲击。随着中国经济地位迅速上升，日本东亚经济主导国的地位受到冲击。从发展趋势上看，似乎中国正在逐渐确立自身在东亚地区的主导地位。日本对这一前景并不是欣然接

受，而是试图延缓这一进程，或者通过与其他国家联合的方式，制约中国影响力的上升。这就在一定时期内增大了双方发生摩擦的可能性。类似的情况，也在中印关系、中韩关系等双边关系中不同程度地存在。 四是美国在东亚地区，存在着一个同盟体系。中国周边的日本、韩国、都是美国的盟国，此外，美国与菲律宾、泰国、新加坡、印度等保持着紧密的安全合作关系。特别是，日美同盟关系在很大程度上是直接针对中国的。美国的战略是维持在全球事务中的领导地位，并试图排除任何一个大国对美国领导地位可能发起的挑战。因此，近来美国的战略东移，试图牵制中国崛起的意图更加明显。 不过，中国的周边环境也存在一些有利因素，首先是中国对周边地区的影响力逐渐增大。中国与周边国家的贸易额巨大，是许多周边国家最重要的贸易伙伴。同时，中国在周边国家的投资也在迅速增长。周边国家对中国经济影响力的扩大，持一种多少有些矛盾的心态，一方面由于中国经济高速增长所带来的经济机会；另一方面，也有一些防范心理，担心从经济上受制于中国。但不管怎么说，中国与周边国家的经济关系，是双边关系中的一个重要稳定因素，这在几年前中日关系的“政冷经热”中已经有清晰的体现。 其次是国际社会对中国战略意图的认知。随着中国的发展，很多周边国家对“中国威胁”感到焦虑。对此，中国提出了一系列政策理念和政策举措，来缓解这方面的焦虑，包括新安全观、和平崛起、负责任大国、建立一系列伙伴关系、提出睦邻、安邻、富邻的外交政策，

意见应以事实为根据. 3 来自辞典例句 192. The bombers swooped ( down ) onthe air base. 轰炸机 突袭 空军基地. 来自辞典例句 193. He mounted their engines on a rubber base. 他把他们的发动机装在一个橡胶垫座上. 14 来自辞典例句 194. The column stands on a narrow base. 柱子竖立在狭窄的地基上. 14 来自辞典例句 195. When one stretched it, it looked like grey flakes on the carvas base. 你要是把它摊直, 看上去就象好一些灰色的粉片落在帆布底子上. 18 来自辞典例句 196. Economic growth and human well - being depend on the natural resource base that supports all living systems. 经济增长和人类的福利依赖于支持所有生命系统的自然资源. 12 1 来自辞典例句 197. The base was just a smudge onthe untouched hundred - mile coast of Manila Bay. 那基地只是马尼拉湾一百英里长安然无恙的海岸线上一个硝烟滚滚的污点. 6 来自辞典例句 198. You can't base an operation on the presumption that miracles are going to happen. 你不能把行动计划建筑在可能出现奇迹的假想基础上.

国际形势之中国周边形势分析 1、当前中国周边安全局势总体评估 第一、周边安全基本形势总体相对稳定 表现在六个方面：第一，中国维护国家安全与周边稳定的能力增强了；第二，地区和平发展的基本态势并没有改变；第三，周边国家对华友好合作仍然是主流；第四，周边国家对中国和平发展的道路认同大于疑虑；第五，发展周边国家友好合作关系的机会仍大于挑战；第六，巩固、发展和周边国家友好合作关系的主动权在我。 第二、消极面增多、矛盾复杂、动荡加剧 近年来，我国周边发生了很多具有冲击力的事件，周边环境恶化的迹象比较明显： 朝鲜半岛局势僵局难破，未来走向不明 从近代开始，以朝鲜半岛为核心的东北亚地区一直是中国安全的致命弱点。 2010年，“天安号事件”和“延坪岛炮击事件”的先后发生，使朝鲜半岛局势出现了1953年朝鲜停战以来最严重的危机，加剧了朝韩之间的紧张关系，韩国李明博政府反映强烈，态度强硬！美国和日本趁机迅速介入，在朝鲜半岛西部和东部海域进行规模空前的军事演习，美、日、韩三边军事同盟雏形初现。 而朝鲜方面，虽有缓和，但仍然坚持核试验，朝核危机阴云不散，六方会谈终止。2011年12月17日，金正日去世，年轻的金正恩接班，半岛局势的不确定性变得更大。 面对东北亚的危机，中国政府游说于美、俄、韩、朝之间，并提议迅速开始六方紧急磋商，但遭到美、韩，日三国的拒绝。中国在东北亚面临着安全威胁与外交压力的双重挑战。 日本“远交近攻”，中国成其主要防范对象 近年来，中日关系虽未有大的起伏，但可以说是“旧仇未了，又添新恨”：中日关系撞船事件态度的强硬、地震之后对待中国给予援助的冷漠、灾难之后美日关系的加强、野田上任之后对防卫重点的调整、第一岛链的设想、与印度的联合军演等等，欲意包围中国。钓鱼岛“国有化”风波未平，安倍晋三又在外交领域显示其“右翼”特征，当然这与其争取国内的选民有关。日本现在经济下滑，国际地位降低，国民焦虑情绪在增加，“正常化”国家的呼声在增加。然而，美国对其仍然是既利用又防范的策略，只要有美国主导的美日同盟在，日本不至于失控。中日关系的关键不在中日之间，而在中美之间。然而，美国对于中国的防范和遏制、拉拢是并存的，在一段时期内中、美、日之间的外交格局不会有大的变化。 第三、非传统安全威胁上升，南亚局势依然紧张 缅甸之变不仅导致难民潮，而且影响到中国在该地区的影响力。 自2008年11月孟买袭击案爆发、印度中止与巴基斯坦的和平进程以来，两国之间时有局部冲突。印巴未能遵守2003年达成的停火协议，在边境地区频繁交火，造成双方武装力量和平民伤亡。2011年5月，印度与巴基斯坦边防军在存在领土争议的克什米尔地区再次发生交火，各有伤亡。印巴关系将长期成为地区稳定的不确定因素。 自2001年美国发动阿富汗战争以来，阿战已进入第12个年头，但阿富汗局势至今无明显好转。相反，随着本·拉登被击毙、美国宣布从阿富汗逐步撤军，该地区新问题、新情况不断涌现。 2、影响中国周边安全形势的因素 影响中国周边安全形势的因素是多方面的，总结起来有三点：美国战略调整是最直接，最明显的影响因素；中国的发展是更深层次的影响因素；中国周边环境的变化是重要因素。 第一、美国战略调整是最直接、最明显的影响因素 导致中国周边环境恶化最直接的因素就是美国的插手。美国之所以插手东边是其战略调整的结果。美国放手“东边”有两个原因，一是“西线无战事”（伊拉克撤军、利比亚问题解决、伊核危机有欧盟、

英语造句大全English sentence 在句子中，更好的记忆单词！ 1、(1)、able adj. 能 句子：We are able to live under the sea in the future. (2)、ability n. 能力 句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地 句子：His calculation is accurately. 3、(1)、act v. 扮演 句子：He act the interesting character. （2）、actor n. 演员 句子：He was a famous actor. (3)、actress n. 女演员 句子：She was a famous actress. (4)、active adj. 积极的 句子：He is an active boy. 4、add v. 加 句子：He adds a little sugar in the milk. 5、advantage n. 优势 句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n.

主语+谓语 1. 理解主谓结构 1) The students arrived. The students arrived at the park. 2) They are listening. They are listening to the music. 3) The disaster happened. 2．体会状语的位置 1) Tom always works hard. 2) Sometimes I go to the park at weekends.. 3) The girl cries very often. 4) We seldom come here. The disaster happened to the poor family. 3. 多个状语的排列次序 1) He works. 2) He works hard. 3) He always works hard. 4) He always works hard in the company. 5) He always works hard in the company recently. 6) He always works hard in the company recently because he wants to get promoted. 4. 写作常用不及物动词 1. ache My head aches. I’m aching all over. 2. agree agree with sb. about sth. agree to do sth. 3. apologize to sb. for sth. 4. appear (at the meeting, on the screen) 5. arrive at / in 6. belong to 7. chat with sb. about sth. 8. come (to …) 9. cry 10. dance 11. depend on /upon 12. die 13. fall 14. go to … 15. graduate from 16. … happen 17. laugh 18. listen to... 19. live 20. rise 21. sit 22. smile 23. swim 24. stay (at home / in a hotel) 25. work 26. wait for 汉译英： 1.昨天我去了电影院。 2.我能用英语跟外国人自由交谈。 3.晚上7点我们到达了机场。 4.暑假就要到了。 5.现在很多老人独自居住。 6.老师同意了。 7.刚才发生了一场车祸。 8.课上我们应该认真听讲。9. 我们的态度很重要。 10. 能否成功取决于你的态度。 11. 能取得多大进步取决于你付出多少努力。 12. 这个木桶能盛多少水取决于最短的一块板子的长度。

当前我国周边安全形势分析 具有5000年文明历史的中国宛如一条巨龙横卧亚洲东方。早在16世纪杰出军事家，政治家拿破仑就曾经说过：“中国是头沉睡的雄狮，未来的某一天将发挥着举足轻重的作用。”事实证明了这位伟人的预言。新中国建国六十余载，其间遭遇坎坷艰辛，但都坦然度过，付出了巨大代价，也收获了宝贵经验。自改革开放战略实施以来，中国这一块古老的土地上又一次迎来了新的活力，改革开放如今已实施三十余年，我国所取得的各项成就足以令世界为之瞩目，我国已经开始了民族伟大复兴的光荣之旅。随着波澜壮阔的二十世纪的结束，世界开始了一个新的纪元，我国也怀着满腔的热情迈入二十一世纪，国富民强，国泰民安的盛世景象开始有了愈加清晰的轮廓。不可否认，我国现在处在一个良好的发展时期，近三十年来，我国创造了举世瞩目的经济奇迹，国家实力进一步提升，这在很大程度上得益于我国三十年间相对稳定的周边环境。众所周知，自70年代对越反击战后，我国基本处于和平状态，无明显对外战争，然而不幸的是，近年来我国的周边环境逐渐恶化，领土、海洋争端不断凸显，周边国家政局动荡，恐怖势力蔓延。而一国的周边环境对一个国家的发展起着非常重要的作用，一个稳定的周边环境是我们寻求和平发展的重要基础，周边形势的好坏将直接影响着我国的国家安全，影响着国家和平发展和战略机遇的实现。 我国是世界上邻国最多的国家，我国疆域辽阔：东临太平洋，西接亚洲腹地，四周分别与东北亚、东南亚、南亚、中亚相邻。回顾历史，冷战结束后，我国与周边国家之间已不存在现实的战争或武装冲突，我国在国土周围也不再面临重大的现实军事威胁。进入二十一世纪后，我国周边政治格局发生了深刻而复杂的变化，总体上是稳定的，有利于和平稳定的因素进一步增强，不利于和平稳定的因素仍然存在并时有激化，总体呈现出机遇与挑战并存的明显特征，而这些都促使我们必须加紧重视周边安全形势。 由于我国邻国最多，因此地缘矛盾最多，最复杂的战略主体与周边国家在领土、领海等权益上，存在巨大问题。中国与周边国家的安全问题就像一条“V形热点线”，呈放射状特点，在北部、西部、南部、东部和东南部都存在着不安全因素。V型线的左端是一条西北东南走向的大陆线，有中亚五国问题，阿富汗冲突、印巴冲突、印中争端等问题；V型线右端是一条东北西南走向的海洋线，有日俄北方四岛之争，日韩的竹岛（独岛）之争、朝鲜半岛之争、中朝黄海大陆架之争、中日东海大陆架和钓鱼岛之争以及南中国海之争等。无疑，这些不安全因素如果不能有效地解除或者处理，将会给我国的经济建设和国家安全造成重大的影响。下面我就中国周围安全形势做一个简单的概括性介绍：

【it's time to和it's time for】 ——————这其实是一个句型,只不过后面要跟不同的东西. ——————It's time to跟的是不定式（to do）.也就是说,要跟一个动词,意思是“到做某事的时候了”.如： It's time to go home. It's time to tell him the truth. ——————It's time for 跟的是名词.也就是说,不能跟动词.如： It's time for lunch.(没必要说It's time to have lunch) It's time for class.(没必要说It's time to begin the class.) They can't wait to see you Please ask liming to study tonight. Please ask liming not to play computer games tonight. Don’t make/let me to smoke I can hear/see you dance at the stage You had better go to bed early. You had better not watch tv It’s better to go to bed early It’s best to run in the morning I am enjoy running with music. With 表伴随听音乐 I already finish studying You should keep working. You should keep on studying English Keep calm and carry on 保持冷静继续前行二战开始前英国皇家政府制造的海报名字 I have to go on studying I feel like I am flying I have to stop playing computer games and stop to go home now I forget/remember to finish my homework. I forget/remember cleaning the classroom We keep/percent/stop him from eating more chips I prefer orange to apple I prefer to walk rather than run I used to sing when I was young What’s wrong with you There have nothing to do with you I am so busy studying You are too young to na?ve I am so tired that I have to go to bed early