DEPOSITION OF HARD CRYSTALLINE AL2O3 COATINGS BY BIPOLAR PULSED DC PACVD

A bipolar pulsed d.c. PACVD technique was used to deposit crystalline Al2O3 coatings onto WC/Co cemented carbide cutting inserts and onto st eel substrates using a gas mixture of AlCl3, H-2, O-2 and Ar. Differen t deposition conditions were tested, substrate temperatures (T-s) betw een 500 and 700 degrees C and a gas mixture ratio O/AlCl3 in the range from 2 to 20. At T(s)greater than or equal to 600 degrees C transpare nt crystalline coatings of the gamma-Al2O3 phase were obtained having the best crystallinity for O/AI ratio of 4 and 6. At a substrate tempe rature of 700 degrees C and an O/AlCl3 ratio of 2, alpha-Al2O3 or a mi xture of alpha- and gamma-Al2O3 phases was formed. The microhardness H V[0.02] of the crystalline coatings was found to be in the range 19-23 GPa. The gamma-Al2O3 layers appeared in most cases transparent with s mooth surfaces. The wear properties of Al2O3-coated cutting inserts pr oduced by the PACVD method were evaluated and compared with a thermal CVD kappa-Al2O3 coating in a machining test in a ball bearing steel. F rom the test results it could be concluded that the PACVD Al2O3 coatin gs showed wear properties very similar to the thermal CVD kappa-Al2O3 coatings. (C) 1998 Elsevier Science S.A. All rights reserved.