Development and evaluation of CrAlAgN self-lubricating coatings for high temperature metal forming dies

Abstract

Conventional lubricants are widely used for die release as well as for cooling assistance on the die surface. However, lubrication is difficult at high temperatures. Oxidation and scaling occur on the work pieces that lead to poor surface finish and a possible warping of the material during cooling. The aim of this research is to develop self-lubricating CrAlAgN nanocomposite coatings for metal forming dies and evaluate their thermal fatigue resistance and wear behavior at elevated temperatures. The CrAlAgN coatings with different Ag contents have been deposited by plasma-enhanced magnetron sputtering. The structure and properties of the coatings were systematically studied to determine the optimal Ag content for achieving a combination of good adhesion, thermal fatigue resistance, and surface lubricity at elevated temperatures. The thermal fatigue resistance of the coatings was evaluated using thermal cyclic testing by cycling the coatings from room temperature to 800 °C up to 1200 cycles. The high temperature wear behavior of the coatings was evaluated using a tribometer up to 900 °C. Good thermal fatigue resistance and low coefficient of friction (COF) were observed in the CrAlAgN coatings with an Ag content in the range of 5–10 at. % at 800 °C. The CrAlAgN coating with 10 at. % Ag exhibited the lowest average COF of 0.05 at 800 °C. The COF of thick CrAlAgN coatings (8 at. % Ag) decreased from 0.5 to 0.2 from 500 to 900 °C, accompanied by an increase in the wear rate under more aggressive wear test conditions. The lubricity of the CrAlAgN coatings at high temperatures was attributed to the lubrication effects from the mixed oxides and encapsulated Ag diffused toward the surface. To further evaluate the coating performance, a hydraulic hot forging punch was coated with a thick CrAlAgN (8 at. % Ag) coating and evaluated in the industrial forging process. The preliminary in-plant trials demonstrated that the coating significantly reduced the dimensional distortion and wear for the forging punch.