Shot Peening Titanium For More Scratch Resistant Matte Finish VS (Glass) Bead Blasting

This article is about shot peening titanium for the purpose of creating a more scratch resistant surface finish, not to improve the mechanical properties like shot peening is normally used for.

Titanium is an excellent material for many applications that require strength, light weight and corrosion resistance, but even in it's strongest and hardest alloy (GR5), it is still relatively soft metal compared to steel and therefore scratches easily.

When using titanium for applications that are cosmetic, like pens, watches, knives, and jewelry, it's difficult to a finish that resists scratches. Polished or brushed titanium will quickly get scratched and show scratches. Bread blasted titanium has a nice matte finish, but will also quickly get scratched when used and start to have shiney parts.

Titanium can be coated in different ways. PVD coatings offer some of the hardest coatings that are used for machine tools to reduce wear. Even the hardest of those coatings, DLC (Diamond Like Carbon) will wear off relatively quickly on titanium. Although the coating is extremely hard, if the underlying material is not hard and dent/scratch resistant, the PVD/DLC coating will get rubbed and scratched off easily, which is what happens when it is applied to titanium. PVD and DLC coatings work best on hardened steel, stainless steel and carbide.

I've tried using DLC on brushed and bead blasted titanium, and it just doesn't last. It lasts longer on brushed/polished finish than bead blasted. I was told by an applicator that the surface roughness needs to be very low to insure proper adhesion. Even glass bead blasting is too rough, and they recommended vapor blasting.

This is the reason that stone washed is the preferred finish for cosmetic titanium products. Stonewashed titanium finish hides scratches and imperfections, because it is made up of random scratches. Stonewashed is a great finish.

SHOT PEENING

Shot peening is much more intense than glass bead blasting and normally used to improve the fatigue life of metal components and prevent cracking. It also hardens the surface of the metal. Shot peening titanium will harden the surface a little and may become more scratch resistant than glass bead blasting.

This forum post had some good info on DIY shot peening.

• #230 cast steel shot is about .032" diameter when new. Cast steel is much heavier than glass bead and therefore imparts more energy to the metal that it impacts.

• 125psi to get .015A Almen arc height. You will need a lot of air volume to keep up the pressure.

• Carbide nozzle with hole size .150 inch (3.81mm)

Cast steel shot is too heavy to be sucked up in the traditional sand blasting cabinet suction setup. It requires the use of a pressure pot setup to blow the steel shot through the line and up to the gun. This also pre accelerates the media before it enters the gun nozzle. Pressure pot blasting equipment is more expensive than suction blasting.

Commercial shot peening is often done using motorized paddles, which is more efficient and consistent than

An alternative way to blast steel shot is using gravity feed. To experiment with the effects of shot peening titanium, I will be using a gravity fed blaster with a small hopper. Using the blaster inside a blast cabinet, the steel shot can be recovered at the bottom and poured back into the hopper. The cabinet vacuum system can be used to remove dust from the air.

The Speed Blaster hopper fed blaster has a ceramic nozzle with 3/16in (4.76mm) orifice. That's a bit larger than the recommended 0.150 inch (3.81mm).

The Le Lematic AS118 blaster has a straight ceramic nozzle, but is still probably 3/16in ID.

The only nozzles I can find smaller than 0.150 inch (3.81mm) are 1/8in, which are mainly available in the cone shaped nozzles.

TO BE CONTINUED...