简介概要

Friction and wear behaviors of Al₂O₃-13 wt% TiO₂ coatings

来源期刊：Rare Metals2013年第1期

论文作者：Lin Lu Zhuang Ma Fu-Chi Wang Yan-Bo Liu

文章页码：87 - 92

摘要：Nanostructured and conventional Al2O3-13 wt%TiO2 coatings were manufactured by air plasma spray. Friction and wear behaviors of coatings were investigated at room and elevated temperatures using an SRV wear test machine. The nanostructured coating has "two regions" microstructure, while the conventional coating has typical layered microstructure with obvious interfaces among splats. The coefficient of friction decreases with rising of temperature because of the formation of tribo-layer at elevated temperatures. The wear resistance of the nanostructured coatings is higher than that of the conventional coating, and the wear threshold of applied load is 30 N for conventional coating and 40 N for nanostructure coating. The wear resistance difference is related to the "two regions" microstructure of nanostructure coating, which could blunt or branch the cracks propagation. In our test ranges, the wear rates rising are more sensitive with the applied wear load rising than with the temperature rising.

Rare Metals 2013,32(01),87-92+2

Friction and wear behaviors of Al₂O₃-13 wt% TiO₂ coatings

Lin Lu Zhuang Ma Fu-Chi Wang Yan-Bo Liu

National Key Laboratory of Science and Technology on Materials under Shock and Impact, School of Materials Science and Engineering, Beijing Institute of Technology

作者简介：Zhuang Ma e-mail:hstrong929@bit.edu.cn;

收稿日期：22 February 2012

基金：financially supported by Chinese Ministries and Commissions project(No.503812);

Friction and wear behaviors of Al₂O₃-13 wt% TiO₂ coatings

Abstract：

Nanostructured and conventional Al2O3-13 wt%TiO2 coatings were manufactured by air plasma spray. Friction and wear behaviors of coatings were investigated at room and elevated temperatures using an SRV wear test machine. The nanostructured coating has "two regions" microstructure, while the conventional coating has typical layered microstructure with obvious interfaces among splats. The coefficient of friction decreases with rising of temperature because of the formation of tribo-layer at elevated temperatures. The wear resistance of the nanostructured coatings is higher than that of the conventional coating, and the wear threshold of applied load is 30 N for conventional coating and 40 N for nanostructure coating. The wear resistance difference is related to the "two regions" microstructure of nanostructure coating, which could blunt or branch the cracks propagation. In our test ranges, the wear rates rising are more sensitive with the applied wear load rising than with the temperature rising.

Keyword：

Al2O3-TiO2 ; Tribo-layer; Wear resistance;

Received： 22 February 2012

1 Introduction

Ceramic coating has wide application prospects in machinery,oil,textile,and chemical industries because of its good performance under wear or corrosion conditions.Alumina based coatings are a good candidate for wear resistance applications[1–6].However,brittleness limits the applications of alumina coatings.To increase the toughness of a coating,different quantities of other ceramics were blended with alumina to maintain a balanced equilibrium between properties.Titanium oxide with low melt point can play a role in binding the alumina grains.Goberman et al.[7]did lots of research on the topic of the alumina and titanium coatings discussing the weight ratio optimization of the two ceramics,and Al₂O₃–3 wt%TiO₂(AT-13)coating was believed to be a good candidate in many situations[7–12]Air plasma spray(APS)is supposed to be an ideal method to deposit AT-13 coating,because APS has several advantages such as easy operation and chemical composition control Nanostructured and conventional coatings of Al₂O₃–TiO₂produced by thermal spraying were investigated in the past years.The nanostructured coatings possess a unique microstructure comprising fully melted regions and partially melted regions,different from the layered structure of completely melted regions in conventional coatings.The nanostructured coating could improve properties,such as higher hardness and greater wear resistance than conventional coating[7,11,12].However,previous studies were limited to wear resistance at room temperature[13];in fact wear resistance at elevated temperature is also important for the industry.In addition,the previous researchers focused on wear resistant properties but paid no attention on friction properties,actually,the coefficient of friction also affects wear process a lot.

In this work,the friction and wear behavior of conventional and nanostructured AT-13 coatings were investigated in load range from 20 to 50 N and in temperatures ranging from room temperature to 500°C using the SRV wear test machine.The wear and friction properties of conventional and nanostructured coating were investigated and compared in each condition,and the wear mechanism was explained through the microstructure and the wear scar of coatings.The variation of coefficient of friction was explained by the presence of tribo-layer.

2 Experimental

2.1 Coating preparation and basic properties

The conventional and nanostructured AT-13 coatings were deposited on 45#steel by Praxair TAFA 5500-2000 plasma spraying equipment with SG-100 spraying gun.The thickness of ceramic layer and bonding layer are 0.5 and0.1 mm,respectively.The nanostructured AT-13 coatings were prepared from the agglomerated powder with diameter distributing in the range of 20–60 lm,and the agglomerated powder was fabricated with the nano particles under 100 nm.The conventional AT-13 coatings were prepared from fused and crushed powder within size range of 20–60 lm.The bonding coating was prepared from NiCrCoAlY powder produced by Praxair.Figure 1 shows the morphology of the AT-13 nanostructured agglomerated powder(a)and the fused and crushed powder(b).Fracture and polished cross section were both examined by scanning electron microscopes(S-3500 SEM produced by Hitachi).

Fig.1 SEM images of agglomerate(a)and conventional powder(b)

Microhardness values of the coatings were measured using the LM700 produced by LECO Company and the load was set on 200 g.The porosity was analyzed through the IA32 software(LECO Company)under optical microscopy.The bonding strength was obtained by the WDW-100 electronic tensile testing machine.

2.2 Wear investigation

The wear experiments were carried out with SRV friction and wear testing machinery(produced by OPTIMOL Germany)in load ranging from 20 to 50 N and temperatures ranging from room temperature to 500°C;in our wear tests the Ball-Plate model(circled in the picture)was applied When the load varied,the temperature was set at room temperature.When the temperature varied,the load was set steadily at 20 N.Besides,the frequency was set at 20 Hz,and the strike distance was 1.3 mm and the testing time was20 min for all the samples.The silicon nitride ceramic balls(U6 mm)were used as the counter body.The wear tests were conducted under dry sliding conditions without eliminating the formed debris.Before the test,the coatings were polished to Ra B 0.5 lm to eliminate the effect of starting roughness difference.Furthermore,the coatings and the counter body were cleaned using ethanol to avoid the presence of humidity and other undesirable films,such as grease.During the test the friction coefficient was recorded simultaneously by the SRV wear test machine.To observe the wear scar clearly,the test specimens were cleaned by ultrasonication in ethanol before the observation.Wear rates were calculated using the mean measurement value of three samples in terms of the volume of the coating material removed per unit load and sliding distance,in unit of mm³?N^-1?m^-1.

3 Results and discussion

3.1 Microstructure

The morphologies of the cross section of the nanostructured AT-13 coatings are shown in Fig.2.Figure 2a indicates the fracture surface and Fig.2b exhibits the polished surface The coating consists of two distinct regions,one is completely melted area(marked by‘‘A’’)formed from completely melted agglomerated powder and the other is incompletely melted area(marked by‘‘B’’)with a particulate nanostructure retained from the starting agglomerates,and there is no obvious layered structure.The microstructure features of the conventional coating are shown in Fig.3.The conventional coating consists of columnar grains and has typical layered structure with obvious interfaces among splats.

Fig.2 Fracture(a)and polished(b)cross section(SEM images)of nanostructured coating

The formation of‘‘two regions’’can be explained as follows:because the nanostructured particle agglomerates from many nano particles,many interfaces exist inside each agglomerated particle.These interfaces block the heat transferring from the outer rim into the inner part of the particle,the exterior of the particle could be well melted but the inner part could not.Consequently,the nano structure is retained within some agglomerated particles,yet the nano particle size increases to 500–900 nm because of the heat effect.By the way,the well-melted exterior leads to good bonding interfaces between depositing particles,so the nanostructured coating has no obvious layered microstructure.The measured microhardness,bonding strength,and porosity of nanostructured AT-13 coatings are HV 956.4,29.85 MPa,and 6.34%,respectively,which were all better than those for conventional coatings(HV763,18 MPa,and 15.98%).

3.2 Friction and wear behavior

Figure 4 shows the variation of friction coefficient of all the coatings along with time.The results show that the time averaged value of coefficient of friction decreases with rising of temperature,while it indicates no rising or dropping trend with rising of load.This dropping trend of coefficient of friction is thought to be related to the third body between the wear pair and the tribo-layer.

Fig.3 The fracture(a)and polished(b)cross section(SEM images)of the conventional coating

Figure 5a–c show the typical wear scars of nanostructure coating obtained at room temperature,200 and 500°C respectively.A lot of smooth and compacted areas(circled areas)dispersed in the wear scar are obtained at 500°C;also,some dispersed in the wear scar obtained at 200°C but no smooth areas could be found in the wear scar obtained at room temperature.It can be concluded that the area of this kind of smooth and compacted region in the wear scars increases with rising of temperature.This kind of region is the so-called tribo-layer,which was also mentioned by Cui et al.[14]and Riahi et al.[15].Since the tribo-layer is compact and smooth,it plays an important role to reduce the coefficient friction.The EDS analysis was carried on the worn surface obtained at 500°C of nanostructure coating;the results are exhibited in Fig.6and Table 1.Two positions are chosen in the wear scar indicated in Fig.6,one is on the smooth and compacted tribo-layer and the other is on the rough surface region Comparing the atomic composition of these two positions listed in Table 1,the obvious difference that can be found is the atomic percentage of silicon element.The atomic percentage of Si element in tribo-layer is 21.35%and that is 11.44%in rough surface area,the atomic percentage of Si element in tribo-layer is much higher than that in the rough surface region.During the process of wear,the silicon element transferred from the Si₃N₄and the oxidation of Si₃N₄took place.It is considered that the oxidation of Si₃N₄affects the formation of the tribo-layer,and the rising of temperature prompts the oxidation of Si₃N₄.The combination of the oxides of silicon,aluminum,and titanium formed the tribo-layer.Hence,as the temperature increases,the area of the tribo-layer regions becomes larger.

Fig.4 Coefficient of friction in different conditions:nanostructure coatings at room temperature(a)and at elevated temperature(c);conventional coatings at room temperature(b)and at elevated temperature(d)

Fig.5 Wear scars of nanostructure coating obtained at room temperature(a),200°C(b),and 500°C(c)

Wear rates of the different Al₂O₃–13%TiO₂coatings are presented in Fig.7.The wear resistance of the nanostructured coatings is higher than that of the conventional coating.As demonstrated in Fig.7a,at room temperature,the wear rate increases with rising applied wear load and there are points of inflexion on both of the curves The inflexion point can be considered as wear threshold value of coating,wear becomes severe when the load is up to threshold value;so in our test,the wear thresholds of applied load are 30 N for conventional coating and 40 N for nanostructure coating.According to Fig.7b,when the applied wear load remains at 20 N,the wear rates also slightly increase with rising temperature but,up to 500°C there is no inflexion point on the curves and the wear rates maintain a low level.It can be concluded that,in our test ranges,the rising wear rates are more sensitive with the applied wear load rising than with the temperature rising.

Fig.6 EDS analysis on tribo-layer(a)and element content(b)

Table 1 Statistical average date of element percentage in the worn surface according from EDS analysis 下载原图

Table 1 Statistical average date of element percentage in the worn surface according from EDS analysis

To explain and verify the difference of wear resistance of coatings,morphologies of wear scars of coatings were analyzed.Wear scars of coatings obtained under 50 N are shown in Fig.8.As indicated in Fig.8a,in nanostructure coating the splats still closely bond together but,in Fig.8b,many splats peel off from the conventional coating.This difference can be attributed to the different microstructures of the two kinds of coatings.The nanostructured coating is dense and with‘‘two region’’features and with no obvious interface,the conventional coating is layered structure and with obvious interface.During the wear process,the interfaces among the splats are always the starting points of peeling off and the cracks generation paths,the unmelted nanostructure regions in nanostructured coating can blunt or branch the cracks and improve the fracture strength of the coating.So the nanostructured coating is not as fragile as the conventional coating.This reduces the opportunity of peeling off of splats.

Fig.7 Wear rates of coatings with different applied load(a)and at different temperature(b)

4 Conclusion

The nanostructured coating has‘‘two regions’’microstructure,while the conventional coating has typical layered microstructure with obvious interfaces among splats.The coefficient of friction decreases with rising of temperature because the formation of a tribo-layer becomes easier a elevated temperatures and the compact and smooth tribolayer plays an important role in reducing the coefficient friction.The wear resistance of the nanostructured coatings is higher than that of the conventional coating,and the wear thresholds of applied load are 30 N for conventional coating and 40 N for nanostructured coating.In our test ranges,the rising wear rates are more sensitive with the rising applied wear load than with the rising temperature.The wear resistance difference is related to the unique microstructure of nanostructure coating,which could blunt or branch the cracks propagation.