Thermally sprayed coatings have been used in various fields of industry for enhancing surface characteristics of materials and extending their service life. The contact surface of some mechanical equipment such as the fine pulverization equipment which is used in the woody biomass production process is required to have wear resistance in the water environment. Thermally sprayed coatings would be a good candidate to improve surface wear resistance under water lubrication. The objective of this study was to evaluate the tribological performance of thermally sprayed coatings under water lubrication. Thermally sprayed coatings which were classified into WC, WB and Ni spraying of three categories were compared with water-lubricated sliding test at a sliding velocity of 0.02 m/s and mean pressure of <i>p</i>0 = 10 MPa with a ring-on-disk apparatus. Thermally sprayed coatings showed comparatively high friction coefficient and well wear resistance under water lubrication. WC contained coatings showed better wear resistance than WB and Ni coatings. Thermally sprayed coatings showed obviously different mechanical properties and tribological behaviors, and the effect of wettability and hardness on tribological characteristics was discussed under water lubrication. Friction coefficient increased as the surface contact angle of thermally sprayed coatings increased. The wear rate decreased as the surface hardness of thermally sprayed coatings increased. Wear resistance of thermally sprayed coatings was excellent under water lubrication. WC contained coatings showed lower wear rate than WB and Ni coatings. WC-14CoCr coating showed the lowest wear rate.
Water lubricant has many advantages such as low negative environmental impact and high cooling capacity. In contrast, water lubricant also has some problems such as high friction and wear due to the low lubricating property and corrosion [
Recently, some new advances in the technology have extended the applications of thermally sprayed coatings in various fields of industry for enhancing surface characteristics of materials and extending their service life [
Thermally sprayed coatings which composed with different powder types would display obviously different mechanical properties such as surface wettability and surface hardness. Due to the low viscosity of water, the wettability is very important for tribological behavior of thermally sprayed coatings under water lubrication. Hardness is one of the most important parameters of thermally sprayed coatings influencing the tribological behavior. Therefore, the mechanical properties of thermally sprayed coatings were characterized, and the effect of wettability and hardness properties on tribological characteristics of thermal sprayed coatings was also discussed. This research may help to understand tribological properties of thermally sprayed coatings under water lubrication, and to select the appropriate thermal spray coatings as the surface of finely pulverized parts which are used in woody biomass production process.
The test disk and ring were made of martensitic stainless steel (JIS SUS440C). The diameter and the thickness of disk were 40 mm and 10 mm, respectively. The inside diameter and the outside diameter of ring were 20 mm and 25.6 mm, respectively. The height of the ring was 15 mm. The specimens were finished by grinding after quenching and tempering, and the surface roughness was approximately 0.01 μm Ra. The surface Vickers hardness was around HV670.
In this paper, WC contained (WC-CrC-Ni, WC-CoCr and WC-Ni) and WB contained (WB-CoCrMo) coatings which were deposited using high velocity oxyfuel spraying (HVOF) and Ni spraying coating were employed and evaluated their tribological properties under water lubrication. The coatings produced by different type powders in thermal spray processes exhibit a broad range of coating hardness, porosity and microstructural features. Thermally sprayed coatings were sprayed on the ring and disk surfaces. The coating specifications were summarized in
WC-1 and WC-2 were formed with WC (Bal.)-CrC (20 mass%)-Ni (7 mass%) powders whose particle size range were 15 - 45 μm and 10 - 38 μm, respectively. WC-3 and W-4 were formed with WC (Bal.)-CrC (18 mass%)-Ni (18 mass%) powders which had lower WC containing ratio than that of WC-1 and WC-2, and their particle size were 15 - 45 μm and 5 - 38 μm, respectively. WC-5 was formed with WC (Bal.)-CoCr (14 mass%) powders whose particle size 15 - 53 μm. WC-6 was formed with WC (Bal.)-Ni (12 mass%) powders which had comparatively large particle size 20 - 53 μm, and had the highest WC containing ratio. WB was formed with WB (Bal.)-CoCrMo (30 mass%) powders whose particle size was 15 - 53 μm. Ni coating was sprayed with high temperature.
Friction and wear properties were evaluated using a ring-on-disk type tribometer. The tests were performed at room temperature under water lubrication. In order to keep the water temperature constant the water was circulated at a rate of 200 mL/min. The ring specimen was installed on rotating shaft and the ring was installed on the loading shaft. The surface pressure was 10 MPa, and the sliding velocity Vs was 0.02 m/s. Total sliding distance in each experiment was set to 10,000 m. In order to observe the change of surface during sliding test, the test was interrupted at an interval of 2500 m and measured the surface characteristics.
The friction coefficient was calculated with the measured friction force and normal force. The wear masses of both disk and ring were measured using the electronic balance with a minimum measurement of 10−4 g. Before and after each sliding test, the ring and the disk were ultrasonic cleaned in an acetone for one hour, then dried in the
Sample code | Powder type | Spraying temperature [K] | Particle size [μm] | Thickness [μm] | Hardness [HV] | Roughness [μm∙Ra] |
---|---|---|---|---|---|---|
WC-1 | WC-20CrC-7Ni | 373 - 473 | 15 - 45 | 160 | 1020 | 0.53 |
WC-2 | WC-20CrC-7Ni | 373 - 473 | 10 - 38 | 160 | 1010 | 0.54 |
WC-3 | WC-18CrC-18Ni | 373 - 473 | 10 - 53 | 150 | 800 | 0.58 |
WC-4 | WC-18CrC-18Ni | 373 - 473 | 5 - 38 | 160 | 750 | 0.44 |
WC-5 | WC-14CoCr | 373 - 473 | 15 - 53 | 160 | 1050 | 0.33 |
WC-6 | WC-12Ni | 373 - 473 | 20 - 53 | 160 | 760 | 0.66 |
WB | WB-30CoCrMo | 373 - 473 | 15 - 53 | 160 | 1000 | 0.79 |
Ni | Ni spraying | 1273 - 1373 | - | 300 | 650 | 0.49 |
vacuum chamber for one hour to remove contaminants.
Due to different powder types and particle size, thermally sprayed coatings showed the obviously different surface characteristics.
analyzed by the SEM equipped with Energy Dispersive X-ray Spectrometer (EDS) shown in the
High magnification images in
The tribological characteristics of eight kinds of thermally sprayed coatings under water lubrication were evaluated by measuring the sliding distance dependency of friction coefficient. The coatings used in this study were classified into three categories: WC metallic coatings, WB metallic coatings, and Ni coatings. The difference in the chemical composition and mechanical characteristics among these coatings were also reflected in their frictional characteristics [
As shown in
decreased with increasing the sliding distance. WC-6 showed the lowest friction coefficient around 0.3 in the eight kinds of thermally sprayed coatings, and it was steady during the test. WB whose coating was sprayed with WB-30CoCrMo showed the highest average friction coefficient value around 0.8. Ni coating showed rather low value of average friction coefficient around 0.55 and a larger fluctuation of friction coefficient during the test. From the test results, eight kinds of thermally sprayed coatings with different chemical composition and mechanical properties showed obviously different frictional characteristics, and most of thermally sprayed coatings showed higher friction coefficients than the substrate material JIS SUS440C. WB sprayed coating showed higher friction coefficient than WC and Ni sprayed coatings. Based on the test results, WC-6 showed the lowest average friction coefficient, and Ni spraying coating showed rather low average friction coefficient. Average friction coefficient of higher Ni containing WC-3 and WC-4 (18 mass%)were lower than that of WC-1 and WC-2 (7 mass%), therefore, it could be speculated that Ni element would play an important role for decreasing friction coefficient under water lubrication. In addition, the larger particles size of thermally sprayed coating showed the lower friction coefficient for the same composition of thermally sprayed coatings.
Thermal spraying is often considered as a potential alternative to traditional coating manufacturing techniques for the production of wear-resistant coatings, and especially ceramic thermally sprayed coatings have been applied to industrial machine parts for anti-wear purposes [
Thermally sprayed coatings are mostly used to enhance service life of the parts exposed to wear.
WC-1 and WC-2 showed smaller wear rate than WC-3 and WC-4 which contained a lower WC. WC-6 showed the largest wear rate in the WC contained coatings. There was a tendency that thermally sprayed coatings contained the higher WC showed the smaller wear rate. In addition, the larger particles size of coatings showed the lower wear rate for the same composition of thermally sprayed coatings.
WC contained coatings also showed obviously different profile curves measured stylus type surface roughness tester and the surface characteristics. In
the test, and this might be caused comparatively greater wear rate and deep grooves on the coating layer. WC-5 with the lowest wear rate showed rather low wear depth and the smooth profile curve, and grinding marks were clearly remained on the surface. WC-6 showed high wear depth and smooth profile curve. In addition, surface roughness of WC contained coatings gradually decreased with increasing the sliding distance. The surface roughness of WC-1 and WC-2 reduced to about 0.2 μm Ra, and those of WC-3 and WC-4 reduced to about 0.4 μm Ra, and those of WC-5 and WC-6 reduced to about 0.12 μm Ra and 0.3 μm Ra, respectively. WB and Ni coatings showed comparatively large wear rate in the employed thermally sprayed coatings. WB showed rather high wear depth but the most smooth profile curve, and little quantity of wear debris was observed on the surface. Ni coating showed rough profile curve and rather high wear depth in the profile curve, and little quantity of wear debris was also observed on the surface. With increasing the sliding distance, surface roughness of WB and Ni coating gradually reduced to about 0.1 and 0.15 μm Ra, respectively.
From the test results, WC sprayed coatings showed better wear resistance than that of WB and Ni coatings, and the more WC containing, the smaller wear rate was shown, and WC-14CoCr coating showed the lowest wear rate. Thermally sprayed coatings showed the tendency that the roughness of wear track gradually decreased with increasing wear. In addition, the larger particles size of coatings showed the lower wear rate for the same composition of thermally sprayed coatings.
The importance of wetting is becoming increasingly obvious and its control is inevitable in many engineering applications including tribology [
showed smaller averaged friction coefficient and a larger fluctuation of friction coefficient than WC-2 which with the same powder. WB which had comparatively low contact angle showed the highest friction coefficient around 0.8. Ni coating which had comparatively low contact angle showed rather low friction coefficient around 0.55. There was a tendency that friction coefficient increased with increasing the contact angle.
Therefore, it seemed reasonable to speculate that the comparatively porous and highly hydrous surface of thermally sprayed coatings caused the rather low contact angle, and it would play an important role for decreasing friction coefficient under water lubrication.
Hardness is one of the most important parameters influencing the tribological behavior, and the hardness of thermally sprayed coatings is basically determined by the powder types. The thermally sprayed coating hardness is presumed to be the main influential factor that dominates wear behavior [
WC and Ni containing ratio together determined the hardness of thermally sprayed coatings.
Based on the above discussion, thermally sprayed coatings showed obviously different tribological characteristics under water lubrication. WC-12Ni coating showed the lowest friction coefficient. Ni element would play an important role for decreasing frictional behavior. The larger particles size of coatings showed the lower friction coefficient for the same composition of coatings. Friction coefficient increased with increasing the surface contact angle. Thermally sprayed coatings showed clearly smaller wear rate than that of the substrate material JIS SUS440C. WC contained coatings showed smaller wear rate than WB and Ni coatings, and the more WC contained coatings showed the smaller wear rate. WC-14CoCr coating showed the lowest wear rate. The specific wear rate decreased with increasing the surface hardness. Furthermore, the surface pressure of fine pulverization parts was about 0.048 MPa in an actual production process of woody biomass, and it greatly smaller as compared with the surface pressure of this test 10 MPa. Therefore, thermally sprayed coatings extremely suited for using on the surface of fine pulverization parts which used in the woody biomass production process.
The tribological behavior of thermally sprayed coatings and their surface mechanical characteristics have been investigated, and the relationship between the test results and surface mechanical characteristics has also been investigated. The main conclusions that can be drawn from the results are the following:
1) The friction coefficient of thermal sprayed coatings was significantly different from each other under water lubrication, and showed comparatively large range from 0.3 to 0.8. WC-12Ni coating showed the lowest friction coefficient. The larger particles size of coatings showed the lower friction coefficient for the same composition of thermally sprayed coatings.
2) Wear resistance of thermally sprayed coatings was excellent under water lubrication. WC contained coatings showed lower wear rate than WB and Ni coatings. WC- 14CoCr coating showed the lowest wear rate.
3) Tribological behavior of thermally spray coatings was obviously affected by mechanical characteristics under water lubrication. Friction coefficient increased as the surface contact angle of thermally sprayed coatings increased. The wear rate decreased as the surface hardness of thermally sprayed coatings increased.
Zhang, C.Q. and Fujii, M. (2016) Tribological Behavior of Thermally Sprayed WC Coatings under Water Lubrication. Materials Sciences and Applications, 7, 527-541. http://dx.doi.org/10.4236/msa.2016.79045