Coatings World - May 2016

carcinogenic hexavalent chromium salts in its production. Increasingly tight restrictions are also being imposed in the United States by OSHA.

A number of Hardide coating variants are available to solve various problems such as wear, corrosion or galling. Coatings are selected based on the individual application and/or operating environment, and can also be tailored to specific requirements. Hardide-A matches the standard thickness ( 50 – 100 microns) and hardness (800 – 1200 Hv) of HCP, simplifying the transition without the need for dimensional changes or drawing re-design. HCP’s intrinsic performance limitations hinder its more demanding wear applications. Hardide-A outperforms it in several key areas including enhanced protection against corrosion, wear, and chemically aggressive media, improved fatigue life and a non-porous structure.

Other alternatives to HCP are available including thermal spray, in particular high-velocity oxy-fuel (HVOF), and emerging processes such as electroless-nickel composite plating, explosive bonding, electro-deposited nanocrystalline cobalt-phosphorus alloys and physical vapor deposition (PVD) coatings. To date, HVOF and other spray coatings have been considered the best available alternative to HCP. Although successful in some applications, each coating has limitations.

Thermal spray coatings can build a very thick and durable layer, but the resultant coatings are rough and porous in structure and often require post-coating grinding which is not possible on intricate shapes. PVD coatings can produce an extremely hard layer with accurately controlled thickness, but they are very thin, typically less than four microns, and have limited load-bearing capacity. However, Hardide-A provides several advantages over HVOF such as the ability to coat complex geometric shapes and internal bores, improved corrosion and fatigue resistance, a smooth as coated low-friction surface and ease of finishing.

The Coating Process

CVD coatings are crystallized from the gas phase atom-by-atom in a vacuum chamber reactor at a temperature of approximately 935o F, producing a conformal coating which can coat internal and external surfaces and complex shapes. The coatings are a metallic tungsten matrix with dispersed nano-particles of tungsten carbide typically between 1 and 10 nanometers in size. Dispersed tungsten carbide nano-particles give the material enhanced hardness which can be controlled and tailored to give a typical hardness range between 800 and 1200 Hv and, with some types of Hardide coating, up to 3500 Hv. Abrasion resistance is up to 12 times better than hard chrome, 500 times better than Inconel and 4 times better than HVOF tungsten carbide.

The CVD coating is applied by a batch process and can be
polished to Ra 0.2 – 0.3 microns ( 8-12 micro-inches) or super-
finished to Ra 0.02 (0.8 micro-inches) without the need for
grinding. This finish does not degrade over time and is a very
effective and ‘friendly’ counterface to seals as it protects metal
shafts or plungers from scratching and scoring that can result
from rotation or reciprocation and which can accelerate the seal
wear. Unlike HVOF, the Hardide coating is free from a cobalt
binder which can be leached from the thermal spray coating in
a corrosive environment leaving a rough and abrasive surface.
As a result, the CVD coated metal counter-surface against which
the seal operates retains a good finish in operation for longer -
even in an abrasive or corrosive environment - and is less abra-
sive for the seal.

When dimensional accuracy is required, the coatings can be diamond ground and super finished for critical bearing surfaces. Hardide coated components being precision measured.