Preparation Of Ti-Al、Ti-Al-Cr Amorphous Composites Coatings By The Method Of Mechanical Alloying And Study Of Crystallization Treatment

The Ti-Al and Ti-Al-Cr amorphous composite coatings were obtained at the room temperatureon the surface of Ti6Al4V（TC4）substrates with the mechanical alloying （MA） method. Furthermore,the amorphous composite coatings fabricated at the optimized parameters were annealed at differenttemperatures. X-ray diffraction （XRD）, scanning electron microscopy （SEM）, energy dispersive X-rayspectroscopy （EDS） were used to investigate the microstructure and chemical element distribution.Multiple testing method aiming to inspect the properties of amorphous composite coatings, includingthe micro-hardness, friction and wear property, scratch and adhesion strength, were completed.The results indicate that the optimum technological parameters of Ti-Al and Ti-Al-Cr amorphouscomposite coatings are as follows: rotation speed is400rpm, milling time is12h. Under this processparameters, the coating thickness of²00μm and¹80μm was obtained. The maximum hardnessvalue of the coatings achieved HV0.1580and HV0.1670. The friction factor was between0.2and0.3.Meanwhile, the critical coating failure load, which can meet the actual conditions of sliding wearapplications, was41.15N and45.1N, respectively.The crystallization experiment under the optimum technological parameters shows that a part ofTi-Al and Ti-Al-Cr coatings was crystallized after the heat treatment at400℃. And the Ti phase andAl₃Ti phase were found in both of the two typed coatings. The amorphous phase can transformcompletely to crystallization after the heat treatment at700℃. Under this condition, the formed Ti3Alphase and TiO impurity phase were found in Ti-Al coating. However, the AlCr₂phase and Al₂Tiphase were predominant crystalline phases in Ti-Al-Cr coating. Microstructure of the depositedcoatings have became more dense and homogenous after heat treatment at700℃. The clear diffusionarea between substrate and coatings were observed, meanwhile, the critical load values of75.2N and70.5N were obtained through the scratch testing. Cyclic oxidation testing at700℃after2h in theatmosphere shows that the oxidation weight gain of Ti-Al and Ti-Al-Cr coatings were3.4mg/cm²and2.6mg/cm², respectively. The resultant values were significantly lower than those of the TC4substratewithout a protective coating.Finally, the synthesis way of amorphous phase, thermodynamics, kinetic conditions and theformation mechanism of Ti-Al amorphous composite coating were further discussed. It indicates thata solid solution forms in the initial stages of mixing of blended elemental powders, and then, when theraw powder is Ti-50at.%Al, it becomes amorphous after the continued milling process. The Ti-Al amorphous composite coatings were built up via a complex combination of cold welding, deformationand fracture processes. The coating thickness increased first, then decreased, and finally verged to aconstant value with the incresement of milling time.