As described above, a typical structure with aluminum splats and stainless steel splats inter-deposited in the HVAS 321/Al coating is formed. Aluminum splats and stainless steel splats have many differences in properties, including hardness, yield strength, elastic modulus and oxidation susceptibility. Once adding a certain content of ductile aluminum particles into the 321 stainless steel coating by the electric arc spraying technique, the quenching stress and thermal stress initiated from the stainless steel particles during the deposition process should be partly released or attenuated. Moreover, as detected by XRD, the oxide in the composite coating is mainly comprised of FeO·Cr2O3 and CrO phases, and the aluminum oxide is so little that can be disregarded. When aluminum wire is used in spraying the 321/Al coating, the mass fraction of the 321 stainless steel particles and oxide will be less than that of the 321 coating. In addition, differences initiated from the spraying method of using two different wires and that of using the dual stainless steel wires may affect the electrode melting and atomizing, thus the oxidizing behavior of the HVAS 321/Al coating changes. As a result, the addition of aluminum in the HVAS 321/Al coating leads to a decrease of oxide content and low level of micro-cracks and residual stresses.
During the sliding friction process, the stainless steel splats act as contacting and load-bearer due to the higher hardness, and the aluminum splats are used to fasten and bond the stainless steel particles, which improves the toughness, shock resistance and delamination resistance of the coating. Furthermore, this type of structure can prevent micro-cracks from extending, because the inter-distributed hard particles of 321 stainless steel will react with the crack tips, thus leading to the deflection of extending and mitigation of wear.
Under the dry sliding conditions, the plastic deformation, plowing and delamination of big splats predominate the wear process of the HVAS 321/Al coating, which results in lower wear resistance than that of HVAS 321 coating. This may due to the fact that the high contact stresses on the coating surface are beyond the limit of bonding strength (cohesive strength) between inter-splats, especially between aluminum splat and stainless steel splat (because aluminum splats and stainless steel splats are quite different in some physical properties and there are no metallurgical reactions between them). That the composite coating contacts directly with the counterbody without any lubricant can lead to plastic deformation and plowing of ductile splats and the delamination of hard splats. In other words, the addition of aluminum material can not play roles under the provided dry sliding condition.
The opposite results for the HVAS 321/Al coating are gained in the lubricated sliding. The lubricated film avoids the direct contact between the surfaces of coating and counterbody, which leads to the reduction of the probability of plastic deformation and plowing of the ductile phases. Furthermore, the contact stresses decrease with the addition of lubricant, which will be very helpful to exhibiting the advantages contributed from the “ductile/hard phases inter-deposited” structure. As described earlier, the content of oxide decreases and amount of micro-cracks initiated from inside of the stainless steel splats are much less than those of HVAS
321 coating, which prevents the stainless steel debris from cracking or disintegrating. Although it is possible that the entire stainless steel splats delaminate from the boundaries of aluminum splats due to the low cohesive strength between the stainless steel and aluminum splats, while compared with the HVAS 321 coating, the delaminating process in the HVAS 321/Al coating needs more energy, e.g. longer fatigue cycle.
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