Enhancing Surfaces: HVAF Tungsten Carbide Thermal Spray vs. Hard Chrome Plating
Hard Chrome Plating has been around in some form since the early 1800’s. It’s popular in many industrial and automotive applications and has been a great surface coating solution for many years. Unfortunately increasing costs, due to environmental regulations, have many people asking, "Is industrial hard chrome plating still the best enhancement solution?" At HTS Coatings, we talk a lot about our HVAF technology and often tout it as the most comparable thermal spray alternative to hard chrome. But when we put them side by side, how do they match up?
Even we will admit that hard chrome has its place, as with most surface protection technologies. For instance, small inside diameter parts, below 3.5 inches, will be more suitable for hard chrome plating. Sometimes, due to the geometry of the part, hard chrome will be more economical. But with advances in thermal spray technology and the increasing environmental pressure against hexavalent chrome, HVAF thermal spray is comparable pricewise and quality-wise with hard chrome plating. So, let’s take a look at how HVAF stands up against hard chrome in the key performance areas of wear and abrasion resistance, cracking, corrosion and friction.
Wear and Abrasion Resistance
The first thing you might think of when determining wear resistance is hardness. On a basic level, the harder the material, the greater the wear resistance. This is technically correct but can also be misleading. For a great explanation of this, check out this article by Superior Consumables. It’s wise to look at a variety of factors and tests to determine wear resistance.
If you look at the graph below, you will see the hardness in Vickers for an HVAF tungsten carbide cobalt chrome (WC-Co-Cr) coating and for standard hard chrome plating. As you can see, not only does an HVAF coating hold its own, but it also offers better hardness depending on the coating material.
We can also look at ASTM G65 wear data for HVAF WC-Co-Cr and Hard Chrome Plating. G65 testing determines abrasion resistance based on how much volume is lost in a controlled load and speed. You can see from the graph that HVAF has less volume loss than hard chrome plating under the standard testing environment.
When it comes to wear resistance HVAF technology is certainly comparable to hard chrome plating and even outperforms it for certain materials and in certain wear environments. So far HVAF coating is comparable to Hard Chrome, but since it is a mechanical coating bond, let’s see how it measures up when it comes to cracking.
Cracking is usually a function of coating ductility and a little bit of bond strength for thermal spray. Ductility can be measured by looking at the young’s modulus of the coating. Not only is this dependent upon the chosen coating material but it can be greatly affected by the coating process itself. The graph of Young’s Modulus, a measure of elasticity, below shows that WC-Co-Cr HVAF coating is more ductile than Hard Chrome plating.
You can also look at something called Fracture Toughness. Fracture toughness uses something called stress intensity factor. It looks at the critical stress level of a sharp crack where the propagation of the crack suddenly becomes rapid and unlimited. Fracture Toughness for WC-Co-Cr and Hard Chrome Plating are roughly the same despite the mechanical bond of the thermal spray coating.
Hard chrome plating has a molecular bond of about 35,000 psi, so it wins out on sheer bond strength. But since HVAF has a greater than 12,000 psi bond strength, it still serves most industrial environments since it is a more ductile coating. Cracking can be an issue for any coating, but corrosion resistance is another key property to look at when comparing HVAF and Hard Chrome plating.
Corrosion is a natural chemical reaction and can occur when parts are in a harsh industrial environment or even just in normal operating conditions. It’s important to know your parts are protected. Corrosion occurs at different rates in different materials but when it comes to coatings, porosity is key. It can be difficult to quantify porosity in hard chrome plating since it is naturally cracked during coating formation. During a salt spray corrosion test, an HVAF tungsten carbide coating passes the 1000-hour test. Hard chrome plating can pass about the 150-hour test, as seen below.
Hard chrome plating can offer some corrosion resistance but if corrosion is your main worry, HVAF will be best. Another reason you might choose Hard Chrome is the surface finish capabilities and how they might affect friction in your system. Let’s see how HVAF and Hard Chrome compare when it comes to friction.
Friction is another key area that hard chrome plating excels at and, until HVAF, cornered the market on. Before, thermal spray required a lot of grinding and polishing to compete with hard chrome which drove the price beyond what was economical. Now, HVAF coatings have an as-sprayed roughness of around 32 Ra, are sprayed thinner and can be easily polished, without grinding, to 0 Ra. This saves time and money and brings the price of finishing into a competitive range with even Flash Chrome.
The Right Solution for You
If you’ve been using hard chrome and it’s been working for you, great, but if you’re feeling the pressure of rising prices or just want to try something different, it may be time to consider other options. Thermal spray, specifically HVAF, can be a cost-effective, comparable solution. Hard chrome will always have its place when it comes to surface solutions and so will thermal spray. Knowing what you need from your surface coatings is the key to determine which of these processes will be best to enhance the life of your machine parts. For more information on thermal spray processes and applications, check out our thermal spray page.