Preparation And Tribological Properties Of D-gun Al₂O₃ Composite Coatings

Detonation gun（D-gun）alumina coating has the advantages of wear resistance,corrosion resistance,oxidation resistance,high bonding strength and high hardness,which has been widely used in the field of tribology.However,the thermal expansion coefficients of the coating and the substrate are quite different at high temperature,and the phenomenon of the coating exfoliation is more serious.Therefore,the high-temperature tribological properties of D-gun alumina coating need to be further studied.Based on the above analysis,Al₂O₃ and Al₂O₃-3wt.%SiC（A3SiC）coatings were prepared by D-gun spraying process in this paper.The mechanical properties and the tribological properties of the coating under vacuum 600°C were systematically studied.The tribological properties of Al₂O₃ coating in molten fluoride salts at 600°C were explored,and the wear mechanism was discussed respectively.The main conclusions were shown as follows:（1）Al₂O₃ coating with NiCrAlY as the bonding layer prepared by D-gun spraying has high hardness of 10～13 MPa,good bonding strength above 51 MPa and low porosity of 0.8%.The tribological properties of the D-gun Al₂O₃ coating under vacuum at 600°C were studied.The tribological properties of the Al₂O₃ coating largely depend on the counterparts.The friction coefficients of Al₂O₃ coatings sliding against SiC ball,T91 pin and Al₂O₃ ball are 0.32,0.24 and 0.48,respectively.Indeed,the wear rates of Al₂O₃ coatings are 20.3×10^-6 mm³·N^-1·m^-1,4.5×10^-6 mm³·N^-1·m^-1,44.1×10^-6 mm³·N^-1·m^-1,respectively.The sliding process of SiC and Al₂O₃ coatings produces a lubricating silica gel.When the T91 slides with Al₂O₃ coatings,Fe in the counterpart transfers to the coating and generates iron oxides,which provides good friction reduction.However,the Al₂O₃/Al₂O₃ tribo-pairs produces amount of abrasive chips in the sliding process.Furthermore,it can be confirmed that the main wear mechanism is abrasive wear.（2）A3SiC coating with NiCrAlY as the bonding layer prepared by D-gun spraying has high hardness of 11～13 MPa,good bonding strength of 43 MPa and porosity of1.92%.With the addition of SiC,the bonding strength of the coating decreases slightly,and the porosity increases.The tribological properties of the D-gun A3SiC coating under vacuum at 600°C were studied.The counterpart is an important factor affecting the tribological behavior of A3SiC coating.The friction coefficients of A3SiC coating sliding against SiC ball,T91 pin and Al₂O₃ ball are 0.22,0.19 and 0.44,respectively.Besides,the wear rates of A3SiC coating are 12.0×10^-6 mm³·N^-1·m^-1,2.4×10^-6 mm³·N^-1·m^-1,28.2×10^-6 mm³·N^-1·m^-1,respectively.Compared with pure Al₂O₃ coating,the friction coefficient and wear rate of A3SiC coating have decreased significantly.Similarly,SiC and T91 pairs can produce lubricating silica gel and Fe oxides,respectively.Moreover,Si element in the coating generates Si O₂,which combines with Al₂O₃ to fill the brittle peeling part of the coating.As a result,the wear of the coating is reduced.The research results indicate that the addition of SiC significantly improves the tribological properties of Al₂O₃ coatings.（3）The tribological behavior of D-gun Al₂O₃ coating in FLi Na K molten salt at600°C was studied.The tribological properties of the Al₂O₃ coating strongly depended on the counterface materials.The friction coefficient of the Al₂O₃ coating ranges from0.14,0.37 to 0.45 by sliding it against SiC ball,T91 steel and Al₂O₃ ball.Accordingly,the wear rates of the Al₂O₃ coating are 2.2×10^-5 mm³·N^-1·m^-1,6.7×10^-5 mm³·N^-1·m^-1and 19.1×10^-5 mm³·N^-1·m^-1 in order.During the sliding,Si element transfers from the SiC ball onto the Al₂O₃ coating surface and forms a compacted Si O₂/Si O_x layer or silica gel,lowering the friction coefficient and protecting the coating from the corrosion and wear in FLi Na K molten.Meanwhile the wear rate of SiC is the lowest.When T91 steel pin is selected,Fe and Cr elements will also transfer onto the Al₂O₃ coating surface and preferably react with the molten salt,alleviating the corrosion and wear of the Al₂O₃coating but increasing the wear of T91 pins.In contrast,the Al₂O₃ coating is confronted with severe mechanical wear and corrosion under the synergism between the hard Al₂O₃ball and molten salt,leading to highest friction and wear of both the sliding pairs.