EVALUATION OF THE ABRASIVE AND THE ADHESIVE MATERIALS RESISTANCE. Michaela Kašparová František Zahálka Šárka Houdková

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1 EVALUATION OF THE ABRASIVE AND THE ADHESIVE MATERIALS RESISTANCE Michaela Kašparová František Zahálka Šárka Houdková ŠKODA VÝZKUM Ltd., Tylova 1/57, Plzeň, CZ, address: Abstract The paper introduces the laboratory apparatus used in ŠKODA VÝZKUM Ltd. that is utilized for the evaluation of abrasive wear resistance of materials. The paper further describes the method for the evaluation of the materials adhesive resistance in accordance with CSN EN 582 (Thermal Spraying-Determination of the Tension Adhesion). The method of the abrasive wear test, called Dry Sand/Rubber Wheel Test (DSRW), is standardized in the ASTM-G65 Standard. DSRW test is characterized by low-stress abrasive conditions and the basic principle of the test corresponds to the injecting of the abrasive sand between the rotating wheel provided with a rubber rim and the tested sample, which is pressed against the wheel simultaneously. Further the Dry Sand/Steel Wheel Test (DSSW) was introduced. The DSSW test method simulates high-stress conditions of the measurement and it is described in the ASTM-B611 Standard. The paper attention is oriented to the changes in the test parameters and to the evaluation of the tested materials and the abrasive sands. Mainly the relationship between the type of the rotating wheel and the wear rate of the tested materials was evaluated. It stands to reason, that the wheel with the rubber rim reduces the abrasive effects. When using a steel wheel the wear of the tested materials increases and also the wear of the steel wheel and the abrasive sands occurs. The shown dependencies were further compared with the used load. For the wear test the following abrasive sands were chosen: Al 2 O 3 of µm grain size and SiO 2, µm in grain size. During the wear test volume losses of the tested materials were measured and the wear track of the tested materials and the abrasive sands morphologies were evaluated using the scanning electron microscopy. The WC-Co and the Cr 3 C 2 -NiCr thermally sprayed coatings were tested. In case of the thermally sprayed coatings the adhesive and cohesive strength between the structural particles and the basic material is the significant factor for the abrasive resistance because of the lamellar structure of the coatings. Therefore the adhesive-cohesive strength of the thermally sprayed coating was performed. Keywords: wear, adhesive strength, thermal spraying, WC-Co, Cr 3 C 2 -NiCr 1. INTRODUCTION The laboratory tests for the wear determination are classified according to the type of the used test apparatus, the main terms that determine the wear levels and the geometric setting of the whole measuring systems [1]. The whole abrasive wear process has typically been divided into two regimes: high-stress or low-stress. If the 1

2 load causes the damage of the abrasive particles the high-stress abrasive wear is concerned and if the damage of the abrasive particles is not distinct the low-stress abrasive wear is concerned. But the difference between the high and the low stress abrasive wear is not strictly specified. The authors in [2] compared the high stress process to the three- or two-body abrasion, where the abrasive particles are fractured and broken apart during the wear process. The three-body abrasion is formed if the small abrasive particles are caught between the two other surfaces and are able to abrade either one or both of the mating surfaces. The abrasive particles are often harder than the two abraded surfaces. The three-body and the low-stress abrasive conditions occur for example in earth moving, mining and minerals industries in a wide variety of items, such as blades, rock drill bits, crushers, ball mills, slurry pumps, etc. The machinery parts are suffering from the progressive wear of their surfaces due to the action of friction and rolling effects of the abrasive fragments that are cleaving between the surfaces of the individual parts [2,5]. The abrasive laboratory tests conditions correlate fast and objectively with the practice. The Dry Sand/Rubber Wheel (DSRW) test standardized in ASTM G65 [3] facilitates to simulate the lowstress/three-body environment. The other laboratory test simulating the high-stress process is called Dry Sand/Steel Wheel (DSSW) test and it is standardized in ASTM B611 [4]. This wear test was developed specially for the WC based hardmetals. In case of the thermally sprayed coatings the abrasive resistance is strongly dependent on the cohesive and the adhesive strength of coatings. Thermally sprayed coatings are widely used for protection of machine function parts against undesirable environment conditions, for example different types of wear, oxidation, high temperature, etc. Thermally sprayed coatings are characterized with their typical properties, which are changing with the used spray technology and deposition parameters. It is mainly the unique microstructure (oxides content, phase composition, cohesion between splats, pores and crack), surface hardness, microhardness, surface roughness, wear resistance and adhesive strength of the coatings. The coatings are bonded with the substrate with the mechanical interlocking above all and partly also with the van der Waals forces and diffusion of the elementary particles on the splats interface. The mechanical interlocking is dependent on the form of the melted particles distribution on the substrate, on the temperature degree of particles melting and on the surface roughness of the substrate. Further, ideal particles location is significantly dependent on the substrate surface topography [6]. It was mentioned in [7,8] that the preheating of the substrate increases the adhesive strength very significantly. It was found that with sufficient substrate preheating the desirable adhesive strength is achieved relatively with low surface roughness. Adhesive strength of thermally sprayed coatings is the important factor, which influences not only the coating properties such as the impact resistance, fatigue life but also its lifetime, applicability and maintenance cost. Therefore, it is necessary to ensure the accurate adhesion strength of a coating system. There are many techniques to evaluate the adhesion strength of coatings [9], such as indentation, shear [12,13] and tensile testing. This paper deals with the tensile adhesion test standardized in CSN EN 582 [10] and in ASTM C633 [11]. 2. EXPERIMENT 2.1 Experimental material For the experiment the thermally sprayed coatings were chosen, cermet WC-Co and Cr 3 C 2 -NiCr. These materials are characterized by a precise wear resistance. The 2

3 WC-Co coating is stable up to the temperature around 500 C and the Cr 3 C 2 -NiCr coating is stable up to the temperature around 850 C. Up to these temperatures the coatings assure the wear resistance. The coatings were sprayed on the steel substrates (ČSN 11373) by means of the JP-5000 gun using the HVOF technology and the optimized spray parameters. The substrates were cleaned and roughened by grit-blasting with 600 µm brown alumina. The surface roughness of the substrate was 9,3±1,1 R a. 2.2 Evaluation of the abrasive wear resistance Abrasive wear resistance was evaluated using the Dry Sand/Rubber Wheel and the Dry Sand Steel/Wheel methods. Using these methods the three-body abrasive conditions are simulated. The result of this test is the materials volume loss in cubic millimetres. At materials of the higher abrasive wear resistance the volume loss is lower. The test principle is schematically depicted in Fig. 1. A brief test description is as follows: the abrasive medium is supplied between the rotating steel/rubber wheel and the tested sample, which is abraded with it. The Fig. 1. The test method in the accordance with ASTM G65 and ASTM B611 Fig. 2. Evaluation of the adhesive strength tested sample is pressed to the wheel by the required load. The materials mass loss is the first test result and it is then converted to the volume loss. Thanks to the values of volume loss it is possible to compare different materials with different densities. The samples were weighted on digital scales with the accuracy of 0,0001g. For the wear tests two different abrasive sands were chosen, the alumina Al 2 O 3 in µm grain size and the silica SiO 2 in µm grain size. The wear tracks in the coatings were evaluated using SEM and EDAX analysis. 2.3 Evaluation of the adhesive strength The adhesive strength was evaluated using the method that is standardized in EN 582 and ASTM C633 Standards. This test method is used for the evaluation of the dependence of the substrate material on the adhesive strength of the coating material, determination of the sample surface treatment before the coating spraying, which influences the adhesion and cohesion, and the evaluation of the coatings adhesive strength. This method is also used for the spraying control [9]. The basic principle is shown in Fig. 2. The tested sample covered on one front side by the coating is joined with the loading and bolstering element using the suitable adhesive. The joining has to be symmetrical and the glued joint has to be loaded by the required force. After the thermal curing of the glued joint the tension test is performed in accordance with EN The tensile strength R H is calculated as the quotient of the maximal load F m and of the sample cross section in the breaking area. The joining was performed using the HTK Ultrabond 100 special glue, which ensures the tensile strength of the glued joint of about 80 MPa. The glue was cured at 180 C per 4 hours in the electric furnace and the failure measurement of the 3

4 samples was performed on the tensile machine at the load velocity of 2mm/min. After the rupture it is evaluated if the glue, the coating or the interface failure appeared. If the glue failure appears it means that the coating adhesive strength is higher than the strength of the adhesive, if the coating failure appears it means the cohesive failure and if the failure in the coating and substrate interface appears it means the adhesive failure. It can happen that the failure is the combination of the last two mentioned cases. In such a case it is the adhesive-cohesive coating failure and the adhesive and the cohesive failure is determined from the fracture area [14]. 3. RESULTS AND DISCUSSION 3.1 Wear resistance and adhesive strength of coatings The results of the DSSW test for the WC-Co and the Cr 3 C 2 -NiCr coating are recorded in Figs 3 and 4. For the WC-Co coating the following facts were found during the DSSW test: using 56N load and the alumina sand already in the third test cycle the substrate was uncovered in spite of the sufficient coating thickness (~350 µm). Using the same load and the silica sand the test was performed for all the test cycles. The coatings wear was only slightly higher than the wear caused by the load of 22N and the alumina sand. Generally, the coatings wear rate can be given in mm 3 /m Fig. 3. Wear of the WC-Co coating Fig. 4. Wear of the Cr 3 C 2 -NiCr coating and the friction coefficient is then converted to mm 3 /Nm. It is obvious that using 56N load and the alumina sand the wear rate is higher by an order than in other measurements. By comparing the friction coefficients it was found that the coefficient of friction was independent of the load and the dependence was ascertained only in the relationship of coating-sand. The value of the friction coefficient varies in the range of 0,001-0,002 for the couple WC-Co/alumina sand and for the couple WC-Co/silica it is lower by an order and equals to 0,0005. For the Cr 3 C 2 -NiCr coating the following facts were found: when using 56N load and the alumina sand already in the second test cycle the substrate was uncovered in spite of the sufficient coating thickness (~350 µm). When using 22N load and the alumina sand the substrate was uncovered in the third test cycle. It was found that the silica sand has higher abrasive efficiency on the load of 56N than the alumina on the load of 22 N. The coefficient of friction is independent of the load and dependent on the used abrasive sand. For both materials it can also be stated that the coatings have the higher coefficient of friction with the alumina sand. The authors in [15] found that the coefficient of friction for alumina is even multiple in comparison with the silica sand. In Fig. 5 both coatings are compared in dependence on the used test method, DSRW vs. DSSW. The results correspond to these measurements conditions: um the alumina sand, 22N load, 718 m of the abrasive length. The results show 4

5 that using the steel wheel causes the multiple wear of the coating material. The steel wheel has a crushing effect mainly on the Cr 3 C 2 -NiCr coating. The wear rate of this coating was nearly identical with the wear of the low carbon steel. The wear rate of the WC-Co coatings increased by about 90% and of the Cr 3 C 2 -NiCr coating by 84%. During the DSSW test many times higher Fig. 5. Wear of the cermet coatings in the comparison of DSSW and DSRW tests, Al 2 O 3 sand, load 22 N stress of abrasive particles on the material is indicated. The negative influence of the hard alumina particles is decreased due to the rubber wheel plasticity at the DSRW test, whereas the steel wheel causes full stress of abrasive particles on the tested material. In the coatings the high stress rises and it focuses on the splats boundaries where low content of pores and hard and brittle oxides occurs. Because of the incidence of high stress together with the action of abrasive particles the splats cohesion decreases and the splats bonding is then defective and the individual splats or their blocks pull out from the coating. Unfortunately this theory cannot be supported by the results of the adhesive-cohesive behaviour of coatings because during the adhesive strength testing only the glued joint failure occurred [14, 16]. In all cases the joint failure was recorded around 80MPa, which is the nominal value of the glue tensile strength. These results only predicate that the cermet coatings sprayed using the HVOF-technology are characterized by the high adhesive strength, which is higher than 80MPa. 3.2 Wear mechanisms Wear mechanism of the Cr 3 C 2 -NiCr coating is documented in Figs 6 and 7. In Fig. 6 the wear track surface is recorded in the low-stress abrasion condition (DSRW test) with the interaction of the alumina particles of µm grain size. The wear track is shiny and in the SEM micrographs the abrasive wear similar to the grinding process can be seen and in the coating the fine grooves appear. These grooves are caused by two main aspects: by the load and by the contact of abrasive particles. The surface is uniformly grooved in sliding direction and uniform abrading of both majority phases - the Cr 3 C 2 carbide and the NiCr matrix - occurs. There is not any visible appearance of the matrix displacement or of some matrix greasing. However the local releasing and the pulling-out of carbides occur consequently (B). Further local process is the carbides cracking in the direction perpendicular to the direction of abrasion (C). The surface of the wear track during the high-stress abrasive process (DSSW) is recorded in the micrographs in Fig. 7. The wear track is matt and it is considerably distorted after using both sands (alumina, silica) and both loads (56N, 22N). From the visual evaluation of the wear tracks relief the significant influence of the load on the surface morphology was not found. It was detected that the track morphology rather changes with the used abrasive sand. The coatings are distinctively grooved and plastically deformed in all directions and the wear track surface is considerably roughened, see Fig. 7a. Higher roughening was caused by using the alumina sand, which was confirmed by the measurement of the wear tracks profiles. Regarding the article limited extent the results of the profile measurements are not listed. Besides the considerable plastic deformation there are also surface areas, in which the pulling out of the whole material block (D), Fig. 7b occurred. The pulled-out material was then carried by the flow of abrasive particles and it 5

6 contributed, together with the abrasive particles, on further grooving of the coating. Unlike the DSRW tests, the abrasive sands debris were entrapped and firmly anchored in the wear tracks (E), Fig. 7b during the DSSW tests. Using the SEM microscopy and the mode of back scattered electrons the anchored abrasive particles can be recognized in the micrographs according to their specific shade of grey. Both types of abrasive sands were detected in the worn surface. During the test the anchored particles could harden the coating surface and act in the coating as hard carbide particles and thus reduced already high wear of the coating. The EDAX analysis confirms the presence of the abrasive particles in the wear tracks. In the SEM documentation the dependence between the load and the amount of the attached sand particles was not found and therefore only the samples loaded by 22 N were put through the EDAX analysis. 23,76% of aluminium and 16,14% of oxygen was analysed during the grinding by the Al 2 O 3 sand. During the grinding by SiO 2 particles only 9,08 % of silicon and 10,78% of oxygen was detected. It indicated that harder and sharper particles of alumina had higher tendency to cleave in the ground surface than the particles of silica. a) b) Fig. 6. The wear scar in the Cr 3 C 2 -NiCr coating: Al 2 O 3 sand, low-stress abrasion, load of 22 N, the wear direction is from up to down, a) scan of secondary electrons, b) scan of back scattered electrons a) b) Fig. 7. The wear scar in the Cr 3 C 2 -NiCr coating: Al 2 O 3 sand, high-stress abrasion, load of 22 N, the wear direction is from left to right, a) scan of secondary electrons, b) scan of back scattered electrons Wear mechanism of the WC-Co coating is documented in Fig. 8, where the wear track surface is recorded under the low-stress abrasion condition (DSRW test) in 6

7 interaction with the alumina particles of µm grain size. The wear track is shiny as in the Cr 3 C 2 -NiCr coating and the abrasive wear is also comparable with the grinding process. The coating is finely grooved (A) and the main wear process is following: removal of a binder phase from the surface layer (C) (this loss of the binder phase weakens the mechanical strength and structure of the surface layers leading to the increased stressing of the WC grains); plastic deformation and grooving of the binder phase; accumulation of plastic strain in the WC grains; fracture and fragmentation of the individual WC grains (D), cracking between the WC grains and pulling-out of the protruding carbide grains (B). Wear mechanism of the WC-Co during the high-stress abrasive conditions was similar to the wear mechanism of the Cr 3 C 2 -NiCr coating. Large surface damage was caused by the alumina particles and the considerable plastic deformation was detected. The wear mechanism caused by the silica particles was similar rather to the low-stress abrasive conditions. Using the EDAX analysis in the wear track it was found that only the alumina abrasive particles cleaved in the surface. 20% of Al was detected in the coatings abraded by the alumina sand and no appearance of Si was detected in the coatings abraded by the silica sand. a) b) Fig. 8. The wear scar in the WC-Co coating: Al 2 O 3 sand, low-stress abrasion, load of 22 N, the wear direction is from up to down, a) scan of secondary electrons, b) scan of back scattered electrons 4. CONCLUSION The main aim of this work was to evaluate objectively the behaviour of the WC-Co and the Cr 3 C 2 -NiCr cermet coatings during the Dry Sand/Rubber(Steel) Wheel tribological tests especially in the relation to their unique microstructure. It was found that these cermet materials, which are known as highly resistant materials under the abrasive wear conditions, drastically change their properties under the high-stress abrasive conditions. The considerable plastic deformation and the detaching of the whole materials block from the surface were found during grinding using the alumina and the silica sand even under the low load of the tested sample. The wear resistance of the Cr 3 C 2 -NiCr cermet almost reached the wear resistance of the low carbon steel. The Cr 3 C 2 -NiCr coating behaved towards both abrasive sands in a similar way whereas the WC-Co coating behaved differently in relation to the used abrasive sand. During grinding by silica sand the WC-Co coating was not so plastically deformed and the deformation was similar to the low-stress deformation under the abrasive conditions. The abrasive wear resistance of coatings is closely related with the cohesive strength between individual structural particles but the measurements did not confirm this theory because during the measurements only the 7

8 glue joint failure occurred at 80 MPa load. The methodology of the evaluation of the coatings in the coating-substrate interface and the strength characteristics between the structure particles (splats) has not been finished yet and it requires a deeper study in the following project stages. ACKNOWLEDGEMENT The paper was prepared thanks to the project No. MSM REFERENCES [1] Mistra, A., Finnie, I. A review of the abrasive wear of metals, J. of Engineering Materials and Technol., 1982, Vol. 104, pp [2] Hawk, J.A., et al. Laboratory abrasive wear tests: investigation of test methods and alloy correlation, J. of Wear, 1999, Vol (2), pp [3] ASTM G65-00, Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus, Annual Book of ASTM Standards, Vol.03.02, United States, 2001 [4] ASTM B611-85(2005), Standard Test Method for Abrasive Wear Resistance of Cemented Carbides, Annual Book of ASTM Standards, United States, 2005 [5] Ma, X., et al. Abrasive wear behaviour of D2 tool steel with respect to load and sliding speed under dry sand/rubber wheel abrasion condition, J. of Wear, 2000, 241 (1), pp [6] Fukanuma, H., Ohno, N. Influence of Substrate Roughness and Temperature on Adhesive Strength in Thermal Spray Coatings, Proceedings of International Thermal Spray Conference, Ohio, USA, ASM International, 2003, pp [7] Watanabe, M., et al. Modified tensile adhesion test for evaluation of interfacial toughness of HVOF sprayed coatings, J. of Surface & Coating Technology, 2008, Vol. 202 (9), pp [8] Bahbou, F., Nelén, P. Relationship between surface topography parameters and adhesion strength for plasma spraying, Proceedings of International Thermal Spray Conference, Ohio, USA, ASM International, 2005, pp [9] Berndt, C.C., Lin, C.K. Measurement of adhesion of thermally sprayed materials, J. of Adhesion Sci. Technol., 1993, Vol. 7 (12), pp [10] ČSN EN 582 Standard, Žárové stříkání stanovení přilnavosti v tahu, Czech Institute of Standards, Praha, 1995 [11] ASTM C633-01, Standard Method of Test for Adhesion or Cohesive Strength of Flame/Sprayed Coatings, Annual Book of ASTM Standards, Vol.02.05, United States, 2008 [12] Siegmann, S., et al. Shear testing for characterizing the adhesive and cohesive coating strength without the need of adhesives, Proceedings of International Thermal Spray Conference, Ohio, USA, ASM International, 2005, pp [13] Hartmann, S., et al. Evaluation of shear test result for determination of shear load resistance of thermally sprayed coatings, Proceedings of International Thermal Spray Conference, Ohio, USA, ASM International, 2008, pp [14] R. Enžl, Vysokoteplotní nástřik povlaků na bázi karbidu wolframu, Dissertation thesis, Applied science faculty, West Bohemia University, 1999, p. 49 [15] Wirojanupatump, S., et al. The influence of HVOF powder feedstock characteristics on the abrasive wear behaviour of Cr x C y NiCr coatings, J. of Wear, 2001, Vol. 249 (9), pp [16] P. Fiala, Vysokoteplotní nástřik povlaků na bázi karbidu chromu, Dissertation thesis, Applied science faculty, West Bohemia University, 2000, p