Wear and Corrosion Protection Using CVD Tungsten Carbide Coatings

Introduction
The life of critical components and tools can be increased by many different hard coatings and surface treatments such as boronizing, nitriding, hard chrome plating, plasma and thermal spray coatings, physical vapor deposition (PVD) and chemical vapor deposition (CVD) coatings. However, each technique has its own disadvantages. This article discusses the innovative CVD tungsten carbide/tungsten coating named as Hardide, Which exhibits exotic properties.
Types of CVD Tungsten Carbide/Tungsten Coatings

CVD tungsten carbide coatings are different from other tungsten carbide coatings as they do not use cobalt or nickel metal matrix binder.
The hardest coating type H comprises pure tungsten carbides, thus demonstrating high hardness but with only 'satisfactory' toughness. The multi-layer coating type M features layers of different hardness. It is possible to modify the overall coating characteristics by changing the ratio between the thickness and properties of individual layer based on specific application requirements. CVD tungsten carbide coating type T is the most widely used coating.
Nano-Structure of CVD Tungsten Carbide Coating Type T

Coating type T is a sophisticated nano-structured material comprising a metallic tungsten matrix with dispersed tungsten carbide nano-particles with a typical size range of 1-10 nm, as shown in Figure 1.
Figure 1. High resolution electron microscopy image of precipitate in CVD coating type T deposited on Cu substrate. The atomic distances (1.49 and 1.76 Å) directly taken from the precipitate region are matched best to the lattice constants of W2C (110- 1.49 Å and 102 - 1.74 Å).

The coating type T demonstrates abrasion resistance up to twelevefold higher than that of hard chrome and improved hardness of more than 1100 Hv. The coating can be formed on titanium, low alloy and some tool steels, stainless steel, and Ni-, Co- and Cu-based alloys. This nano-structured material shown unprecedented impact resistance, crack resistance and toughness by tolerating 3000 microstrain deformations without any deterioration.
CVD Tungsten Carbide Coating Porosity

CVD tungsten carbide coating is devoid of through porosity from a thickness of below 1 µm thanks to its deposition mechanism, as illustrated in Figure 2. The crystallization of the coating takes place from the gas-phase atom-by-atom, and the defects and micro-pores in the coating are filled by the highly mobile reaction products as the coating grows.
Key Characteristics of CVD Tungsten Carbide Coatings

The key characteristics of CVD tungsten carbide coatings are as follows:
Outstanding chemical and corrosion resistance
Free from through porosity, thus effectively protecting mild steel substrate from corrosive media without sealing the coating
Superior resistance to sulfide stress cracking
Provides effective protection against mineral acids such as sulfuric acid and hydrochloric acid
Resists Aqua Regia at room temperature
Superior wear resistance; Wear rate of CVD tungsten carbide coatings is four fold lower when compared to thermal spray WC, 12 fold lower than hard chrome, and 40 fold lower when compared to abrasion resistant steel AR-500
Demonstrates better erosion rate when compared to chrome carbide weld overlay, hard chrome, white iron, cemented carbide, and various other hard materials
Better fatigue resistance and toughness
Deposition from the gas phase enables coating of internal surfaces and intricate shapes.