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ADHESION / DURABILITY COMPARISON -
A qualitative measurement of hardcoating durability and adhesion can be made using a Rockwell "C" indent test (VDI Guideline 3198 Procedure). Coatings are deposited on a polished, hardened steel coupon (Rc> 60) to a thickness of between 1 and 3 microns. A standard Rockwell diamond indenter is used to indent the surface for a "C" scale measurement (150 Kg). This steel coupon is similar in hardness and material type to the precision pharmaceutical tooling that can benefit from wear-resistant coatings, including punches and dies, tamping pins, and compacting rollers.
The indented area is examined at a magnification of 200X and the cracking pattern in the coating is observed. Hardcoatings with good cohesion show little or no fracture lines extending radially from the center of the indent outward towards and beyond the perimeter of the circular indent. Also, no cracked islands appear in the indent crater or along the crater perimeter. Hardcoatings with good adhesion show no delamination of the coating, either in the crater or adjacent to the coating perimeter.
The results of the VDI-3198 test for all four coatings tested were varied:
HARD CHROME PLATING / ELECTROPLATING -
There were radial fracture lines following testing, indicating cohesive failure of the chrome plating / electroplating on the conical indent surface. Most of the radial fracture lines in the coating extend beyond the indent perimeter. A number of cracked islands were formed, but there is no delamination of the coating, either in the indent or adjacent to it, indicating good adhesion under conditions of high stress.
PVD DEPOSITED DIAMOND-LIKE CARBON (DLC) -
There were many radial fracture lines and cracked islands within the conical indent zone, indicating poor cohesion overall. Many of the cracked islands were delaminated from the surface and there was complete delamination of the DLC coating beyond the conical indent, indicating poor adhesion under conditions of high stress.
PVD DEPOSITED TITANIUM NITRIDE (TiN) -
There was a high density of radial fracture lines within the conical indent and beyond the indent perimeter, indicating poor coating cohesion and high coating friability. The presence of cracked coating islands in the conical indent and at the indent perimeter margin, many of which had delaminated, indicated marginal adhesion to the hardened steel substrate under conditions of high stress.
IBED-DEPOSITED TITANIUM NITRIDE (TiN) -
The presence of radial fracture lines was minimal with tearing of the coating on the conical indent surface, indicating excellent cohesion within the coating. Some radial fracture lines with very slight tearing of the coating were seen beyond the indent perimeter. Darkened areas on the conical indent were indicative of dirt particles transferred from the diamond indenter to the indented surface. The absence of multiple cracked coating islands indicated excellent adhesion to the hardened steel substrate under conditions of high stress.
The abrasive wear-resistant performance of coatings can be tested using a Taber Abraser. Performed according to standard procedure, SAE/AMS-2438A (SAE International), coatings are deposited on 3.75 inch (9.5 cm) diameter disks that are rotated against resilient rollers volumetrically impregnated with 50-micron diameter alpha-phase aluminum oxide grits. The coated disks are weighed, run for a fixed number of cycles and then re-weighed. The thickness of coating material worn away can then be calculated. Since standard test parameters are used - governing grit size, wheel RPM and surface loading - the wear rates obtained are directly comparable as measures of how effectively each coating would reduce the wear-rate of pharmaceutical tooling.
All four coatings in the study were deposited on 3.75 inch (9.5 cm) diameter, hardened (Rc 64-66) high speed steel disks that were lapped to a highly polished finish of 0.025 micro-meter RA (1 micro-inch AA). The coatings were deposited to equivalent thicknesses, approximately 4 microns. The results of the abrasive wear test for all four coatings were compared to the wear rate of S7 tool steel hardened to Rockwell "C" 60.
- The wear rate measured for hardened tool steel (S7) was 1.3 microns per 10,000 revolutions.
- Industrial chrome plating / electroplating showed a wear rate of 0.6 microns per 10,000 revolutions - or, 2 times less wear-rate than un-coated steel.
- PVD-deposited diamond-like carbon (DLC) coating showed a wear rate of 0.04 microns per 10,000 revolutions - or, 32 times less wear-rate than un-coated steel.
- PVD and IBED deposited titanium nitride (TiN) coatings both showed wear rates of 0.015 microns per 10,000 revolutions - or, 86 times less wear-rate than un-coated steel.