Study On The Design And Wear Mechanism Of Nickel-based Self-lubricating Composite Coatings Served In Wide Temperature Range

The components operated in aerospace,nuclear energy and other equipment have great demand for the materials with excellent high temperature lubricating and wear resistance in a wide temperature range.The lubricating and wear resistance of components in a wide temperature range ensured the reliability of the entire system.In this paper,Cu,h-BN,MoO₃ solid lubricants and c-BN reinforcements were selected to compound in the nickel-based Ni60 coating according to their characteristics;lubricants and reinforcements were added gradually into the Ni60 coating by laser cladding.The effect of the added lubricants and reinforcements on the miscrostructure and wide range temperature hardness of the composite coating were investigated.The enhancement mechanisms of the composite coatings were clarified.The friction and wear properties over wide range of temperature were investigated to reveal the wear and failure mechanisms of the composite coating.Finally,nickel-based self-lubricanting composite coatings Ni60+Cu/h-BN+MoO₃ and Ni60+Cu/c-BN+MoO₃ with excellent friction and wear properties over wide range of temperature(25°C to 800°C)were successfully prepared.The microstructure characterization of the composite coatings showed that h-BN solid lubricant existed in the Ni60 coating in its original morphology.Most of the added h-BN spattered during laser cladding process,which reduced the content of h-BN in the coating.Compared with h-BN,Cu/h-BN has better compatibility with Ni matrix,which decreased the spatter of h-BN during laser cladding,and therefore increased the content of h-BN from 1.4vt.%to 3.9 vt.%.Further adding MoO₃ into the coating found that MoO₃with low melting point can preferentially melted to form a thin film on the unmelted powder during laser cladding,effectively reduced the spatter,improved the deposition efficiency of the coating and the stability of the laser cladding process.The thickness of the coating increased from 0.94 mm to1.43 mm,the content of h-BN in the coating further increased to 8.4%.It was found that the added MoO₃ reacted with C to form Mo₂C during laser cladding process,and part of Mo atoms dissolved in the Cr(Mo)B reinforcements.The content of the reinforcements in the coating from 13.8vt.%to 21.9 vt.%.c-BN particles partially decomposed during the laser cladding process,B and N elements reacted with the Mo and Cr elements to produce smaller and more uniform distributed reinforcements in the coating.The outer layer of remained boride nitride particles were converted to h-BN lubricant,and the center were still the c-BN reinforcement.Compared with Ni60+Cu/h-BN coating,the addition of MoO₃ and c-BN changed the content,size and morphology of the reinforcements in Ni60+Cu/h-BN+MoO₃ and Ni60+Cu/c-BN+MoO₃ coatings.The main strengthening mechanism in the coating was dispersion strengthening caused by the change of the reinforcements.Followed by the solid solution strengthening of Mo atoms.The effect of dislocations caused by the difference in thermal expansion coefficient and elastic modulus on the strengthening was very little.The hardness results over 25°C,200°C,400°C,600°C and 800°C wide temperature range showed that with the increase of temperature,the hardness of the composite coating decreased gradually.The addition of Cu and h-BN solid lubricants significantly reduced the hardness of the Ni60 coating at lower temperature range.At high temperature,the effect of Cu and h-BN solid lubricants on the hardness of Ni60 coating was small.Compared with Ni60coating,the hardness of Ni60+Cu/h-BN coating at 25°C,200°C,400°C,600°C and 800°C decreased 28%,23%,24%,16%and 5%respectively.The addition of MoO₃ counteracted part of the softening effect of the lubricants.Compared with Ni60 coating,the hardness of Ni60+Cu/h-BN+MoO₃ coating increased 11%,3%and 2%respectively at 25°C,200°C and 400°C.At600°C and 800°C,the hardness of Ni60+Cu/h-BN+MoO₃ increased 6%and5%compared with Ni60 coating.The addition of c-BN completely counteracted the softening effect of the lubricants,and further improved the hardness of the coating over a wide temperature range.Compared to Ni60coating,the hardness of Ni60+Cu/c-BN+MoO₃ coating increased 0.1%?3%?5%?12%and 15%respectively at 25°C,200°C,400°C,600°C and 800°C.The friction and wear properties of the composite coating at 25°C,200°C,400°C,600°C and 800°C were measured.It was found that compared with Ni60 coating,the friction coefficient of Ni60+Cu/h-BN+MoO₃ coating at these temperature decreased from 0.56,0.48,0.42,0.37,0.4 to 0.44,0.42,0.35,0.32,0.37 respectively;the wear rate decreased 20%,26%,49%,32%and 21%respectively.The friction coefficient of Ni60+Cu/c-BN+MoO₃coating at these temperature decreased to 0.45,0.43,0.35,0.29 and 0.33respectively;the wear rate decreased 38%,40%,64%,56%and 39%respectively.The worn surfaces morphology of the composite coatings from25°C to 800°C showed that with the increase of the wear temperature,the wear mechanism of the coating gradually changed from abrasive wear to adhesive wear.The worn surface began to oxidize at 200°C,the oxidation of the worn surface became serious as the wear temperature increased.Cu and Mo on the worn surface reacted with Oxygen in the air to form CuO,MoO₃,and Cu MoO₄ solid lubricants at 600°C and 800°C.The nanoindentation test results revealed that the addition of MoO₃improved the strength and elastic modulus of the reinforcements in the composite coatings.However,c-BN reduced the strain homogenization ability of the reinforcements in the coating.The mismatch rates of the phases interface in the coating were calculated and found that the binding energy of Cr(Mo)B and?-(Ni,M)in Ni60+Cu/c-BN+MoO₃ coating was greater than that in Ni60+Cu/h-BN+MoO₃ coating.Therefore,the first failure position of Ni60+Cu/h-BN+MoO₃ coating during wear process was the interface of reinforcements and matrix,and Ni60+Cu/c-BN+MoO₃ coating was the fragmentation of the reinforcements.Compared with Ni60 and Ni60+Cu/h-BN coatings,the abrasive wear and adhesion failure caused by these two failure mechanisms were relatively minor.After a period time of wear,the worn surfaces formed stable friction interface layers and dense glayze layers,which effectively protected the worn surface and reduced the friction coefficient.