Study Of Nanocrystalline W Coatings With Low Nitrogen Prepared By High Power Impulse Magnetron Sputtering

Hardness and toughness seem as a pair of conflicts in the research and application of vapour deposition hard coatings:the improvement of one side is at the expense of the other one.To solve this problem,highly hard yet toughened and well adhesive nanocrystalline W?N?coatings were prepared on YG6 substrate by doping small atom N using high power impulse magnetron sputtering?HiPIMS?.The effects of working pressure on the structure and properties of nanocrystalline W?N?coatings were systematically studied.At the same time,the structure and properties of WNx coatings prepared by high power impulse magnetron sputtering and in-situ direct current magnetron sputtering were also preliminarily explored.Compared with W?N?-DCMS coating prepared by direct current magnetron sputtering,smoother surface and denser cross-sectional structure of W and W?N?coatings were obtained by high power impulse magnetron sputtering,but the deposition rate is only 30%of that of W?N?-DCMS coatings.The surface morphology of W,W?N?,W?N?-DCMS coatings are as slender rod-like,compact dimple-like and melon seeds-like with gaps,respectively.TEM results show that there is high density stress field in the three coatings and a amorphous layer with a thickness of tens of nanometers at the interface between coatings and substrates.Near the amorphous layer,W?N?coating is a nanocomposite structure consisting of nanocrystalline alpha-W?N?phase embedded in amorphous matrix.The grain size of W?N?coating is as low as 7.5 nm.W?N?coatings exhibits highly hardness?29.4 GPa?,well toughness and adhesion?Lc2>100 N?,also with excellent tribological properties and good corrosion resistance.Compared with W coatings,the wear rate of W?N?coating decreases by an order of magnitude to 5.08×10^-1616 m³/N·m and the main wear mechanisms are oxidation wear.The corrosion resistance decreases by nearly one order of magnitude compared with W?N?-DCMS coating.No corrosion morphology was observed under scanning electron microscope,while the pitting corrosion of W?N?-DCMS coatings occurs obviously.The deposition rate of W?N?coating made by HiPIMS improved with the increase of working pressure;the cross-sectional structure changes from featureless structure with fine columnar grains to columnar crystals with the increase of working pressure.The hardness changes with the structure,decreasing from 33.3 GPa at 0.40 Pa to 25.2 GPa at 0.73 Pa.Excellent adhesion was found on YG6 substrate,for applying 150 N of scratch test yet no failure was observed.Wear mechanism of W?N?coatings at 0.50 Pa against different wear balls was particularly studied.It is found that self-lubricating Magneli phase WO₃ was formed when W?N?coatings worn against Si₃N₄ and Al₂O₃ balls at room temperature.W?N?coatings has the best wear characteristics against Si₃N₄ ball,the friction coefficient is 0.47and wear rate is 4.97×10^-16m³/?N·m?.Wear mechanism of W?N?coatings against Si₃N₄ ball is oxidation wear,while against Al₂O₃ ball is oxidation wear and abrasive wear.The sliding against 304 ball did not wear W?N?coatings.The transferred material from the ball adheres to the coating surface impeding its wear but causing further damage to the ball.High power impulse magnetron sputtering and in-situ direct current magnetron sputtering were also preliminarily explored.Transition layer deposited by HiPIMS has template effect on the following DCMS coatings,and the surface topography changes from"onion-like"to the angular"melon seed-like".The duplextreated coatings are composed by?-W and a small quantity of W₂N,with the hardness of 26.4 GPa,slightly higher than monolayered coatings.Furthermore,the wear mechanism of duplextreated coatings against Si₃N₄ ball under different loads is affected by the oxidation degree of wear debris.The best wear characteristics were observed under 10 N with severe oxidation wear,while oxidation wear,abrasive wear under 5,15 N.