Diamond coatings were deposited on WC-Co cutting inserts by microwave plasma enhanced chemical vapour deposition. To restrict the catalytic effect of cobalt binder phase for graphite formation, a combination of hydrochloric acid etching and H_2 plasma treatment was used as substrate pretreatment to reduce surface Co concentration and modify substrate surface morphology. SEM results show that continuous nanocrystalline diamond coatings were formed on pretreated substrates, whereas a composite of diamond and graphite was formed on untreated substrates. Raman spectra show strong diamond characteristic peak at about 1332cm"1 on pretreated substrates, while only strong graphite peaks and weak diamond peak are on untreated substrates. No film spallation was observed along the scratching track in scratching tests of the diamond coatings on pretreated substrates. Among the three tested pretreatments (HCl+H_2, H_2+HCl, HCl), H_2+HCl, hydrogen plasma etching followed with hydrochloride etching produces the highest diamond phase content.
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