Study On Microstructure, Mechanical Properties And Superhardness Effect Of CrA1N-based Single Layer And Nanomultilayer Coatings

The CrAlN coatings are raising a widely interesting by its excellent properties,such as mechanical properties, high temperatuter oxide resistance. Therefore, the DCreactive deposited process and Al/Cr atomic ratio effect on CrAlN coating’s structureand property were studied in this paper. Furthermore, we brought forward strategy tosynthesize a new kind of hard coating, synthesizing the ternary nitride/oxidenanomultilayer by reactive methods. CrAlN/SiO₂and CrAlN/AlON nanomultilayerswere prepared, and the microstructure and mechanical properties were characterizedand measured by EDS, XRD, SEM, HRTEM and nanoindentation. The frictioncoefficient, and wear scar were measured for the sample with different Al-contents.The experiments results were explained by the present theories.The main conclusionswere summarized as follows:（1） Process parameter effects on the coating: The results indicated that the DCmagnetron sputtering deposition can be used to prepare compact-grain structure, highquality CrAlN coatings with c-CrN phase at a high deposition rate. Low Ar/N₂flowrate could add the N-content of the coatings and then improve the mechanical property.But too much nitrogen would lead to decrease of the deposition rate and propertybecause the target was covered by nitrides. The best coating was deposited at Ar/N₂flow ratio of1. Total gas pressure influences the number and energy of depositionparticles. Mechanical properties of coatings decrease with increase of the pressure. Thedeposition rate increases firstly then decreases. The grains grow at high temperaturewhile the defects increase at low temperature, which would be harmful for themechanical properties. The maximum hardness and elastic modulus could reach34.8GPa and434.3GPa respectively under the optimized process.（2） Al contents effects on the coating: the XRD results showed that the coatingsexhibited mainly NaCl structure. When the Al contents were between30%and60%,the coating contained only c-CrN phase, while the Al content was70%, the coatingcontained both c-CrN and c-AlN phase. The lattice parameter decreased gradually withincrease of Al content, while the grain size firstly decreased then increased withincrease of the Al content. With the solution strengthening, grain refinementstrengthening and formation sufficient ceramic phase effects, the comprehensiveperformance firstly increased and then decreased with the increase of Al contents, the excessive Al leaded to phase transformation and N insufficientce, which decreased theproperties. The maximum hardness and elastic modulus reached36.8GPa and459.5GPa respectively, and minimum abrasion loss when Al content was about60%.（3） CrAlN/SiO₂nanomultilayers with various SiO₂layer thicknesses weresynthesized by reactive magnetron sputtering. The results revealed that, thenanomultilayers had the same phase with CrAlN. Under the template effect of CrAlNlayer, amorphous SiO₂was forced to crystallize and grew epitaxially with CrAlNlayers when SiO₂layer thickness was below0.7nm. The interface of two layers grewexpitaxially and existed modulus difference, which resulted in increase in hardness andelastic modulus, and the nanomultilayer appeared superhardness effect with themaximum hardness and elastic modulus of38.9GPa and417GPa respectively whenSiO₂layer thickness was0.7nm. As the SiO₂layer thickness further increased, SiO₂layer transformed into amorphous state and broke the coherent growth of multilayers,resulting in the decrease of hardness and elastic modulus.（4） CrAlN/AlON nanomultilayers with various AlON layer and CrAlN layerthicknesses were synthesized by reactive magnetron sputtering. The results revealedthat, under the template effect of CrAlN layers, amorphous AlON was forced tocrystallize and grew epitaxially with CrAlN layers when AlON layer thickness was0.9nm, and the nanomultilayer reached the maximum hardness and elastic modulus of32.8GPa and404GPa. Throught the expitaxilly coherent growth interface was formed,there was no observation in hardness improved significantly because of no enoughmodulus difference between two layers. As the AlON layer thickness further increase,AlON transformed into amorphous state and broke the coherent growth of multilayers,resulting in the decrease of hardness and elastic modulus. The hardness increaseslightly with decreased of CrAlN layer and became stable when the CrAlN layerdecreased to1.9nm, value of34.7GPa.