Investigation of the adhesion behavior of aluminum on various PVD coatings applied to H13 tool steel to minimize or eliminate lubrication during high pressure die casting, An

Abstract

The development of hard coating compositions and architectures that reduce or eliminate the need to lubricate die casting dies, core pins and inserts during high pressure die casting has been investigated in detail. Initially, two conventional methods for determining wetting and adhesion properties were examined, namely, the dipping test and the sessile drop test. It is shown that neither of these tests generated reliable wetting/adhesion results. For the dipping test, the aluminum adhesion to uncoated steel pins was strongly influenced by the physical shrinkage of the aluminum alloy upon solidification and cooling. For the sessile drop tests, the contact angle could not be measured accurately due to the tenacious oxide scale on the liquid aluminum surface, which prevents the aluminum from spreading and wetting the substrates; this was the case regardless of the environment (inert gas or high vacuum) used. Based on the ineffectiveness of these conventional tests, a new test, referred to as the aluminum adhesion test (AAT), was developed with the goal of quantitatively measuring the aluminum adhesion to various uncoated and PVD-coated H13 steel substrates. The coatings that were tested using the AAT were divided into four groups: (1) those that had no adhesion strength (e.g., AlCrN, AlTiN and CrWN), (2) reactive metals (e.g. chromium) that either dissolve into the liquid aluminum or react to form intermetallics, (3) those with surface properties that result in nanoscale mechanical interlocking that result in the highest aluminum adhesion (e.g., TiB2), and (4) those with intermediate adhesion strengths. This latter group includes most of the ceramic coatings having dense microstructures, which, although chemically inert, exhibit a range of adhesion strengths. It was shown that, for all of the non-sticking coatings studied, the AlCrN coating appeared to be the best candidate because of its oxidation resistance at and above typical die casting temperatures. Consequently, the sticking behavior of various AlCrN coatings was examined as a function of coating chemistry (Al/Cr ratio), surface oxide layer, and surface roughness. It is shown that the Al/Cr ratio is less important than surface roughness/topography. Finally, in-plant die casting trials were conducted on the optimized AlCrN coatings and the results of these trials will be described in some detail.