Surface engineering and golf club
|Surface engineering technologies offer a wealth of opportunities for golf club design to enhance performance and playability. Lasers are particularly well suited to processing golf clubs due to their flexibility, speed, and cost. Advanced materials have also played a dramatic role in the evolution of golf clubs. The last five years have shown titanium alloys to be the (almost) undisputed material of choice for the construction of driver heads due to its high specific strength and recent improvements in cost-effective manufacturing overseas.
In the last couple of years, Ti–6Al–4V and b-Ti face drivers have dominated the industry due to the ‘spring-like’ effect in drivers with very thin faces (typically less than 1.2–2.0 mm) where the coefficient of restitution (COR), a measure of the efficiency of the collision between the ball and club, can be increased to a previously unattainable level. The bene t is so signicant that the governing body of competitive golf in the US, the United States Golf Association (USGA), decided to limit the COR to 0.830 mm (z/2 manufacturing tolerances). Although the USGA’s limitation has reduced the possibility of further developments in this area, there are many other technologies open to club manufacturers. One such area, surface engineering, is of particular interest as it is relatively unexplored in the field of golf club research.
Laser surface modification
The laser surface modification (LSM) process utilizes a high power laser to functionally modify the hitting surface of a golf club. Yttrium aluminum garnet (YAG) lasers are particularly well suited to the surface modi cation of golf clubs due to the high absorption of the laser energy at 1.06 mm for many of the alloys (e.g. Ti 6Al–4V, 1020 steel, 17–4PH, 431 stainless steel) used in golf club construction.
Titanium and its alloys are well suited to LSM due to a high reactivity with gaseous species which can be controlled to provide the formation of a high modulus and high hardness compounds such as TiN and TiC.1–5 Many interesting surface morphologies have also been generated with different potential performance improvements for golf clubs and they are the subject of ongoing research.
There are many different experimental variables contributing to differences in a surface texture with laser processing. Some of the important ones include pulse period, pulse width, dwell time, laser power and focus. Experiments to date have shown a perfectly focused beam can produce an extremely high degree of heating resulting in evaporation of surface titanium with very little heat input to modify the bulk of the material. For current golf club applications, several defocused conditions are used to provide the ne surface texture in addition to a small amount of subsurface annealing to reduce residual stresses resulting from the forging process.
Alternative surface engineering processes
Chemical vapour deposition (CVD) and physical vapour deposition (PVD) are techniques that deposit a very thin coating (typically 1–5 mm) on the surface of a material. Thin lm coatings have been used in golf applications but primarily for aesthetic reasons. Many of the metal nitride and carbon based coatings have particularly attractive colours ranging from a light silver to a deep black. The coatings work well in the case of putters, which have low impact velocities and limited contact with abrasive materials in the soil. In the case of wedges, irons, and drivers, however, the coatings have a limited lifetime due to wear and spallation of the section of the club that comes into contact with the ground.
|Thick film coatings, which generally range 5–500 mm, can be deposited using a number of different processes with thermal spray as one of the most common methods. Plasma spray and thermal spray coatings have also been used in golf applications, but primarily to apply a hard, wear resistant coating to the impact face of a club. This technology allows a coating to be made from almost any material that can be made into a powder or wire and fed into the hot zone of the plasma spray. However, many such coatings are rough and can require post-deposition finishing.|
|Computer numerical controlled (CNC) milling has become increasingly popular for the production of high end, high priced putters due to the tight dimensional tolerances and the excellent fitness achievable on the striking face of the club.6 In addition, several golf club manufacturers have moved towards CNC milling on the face of wedges and irons in order to ensure flatness that is not achievable through the casting or forging process used to form the club. While many people may not view CNC milling as a surface engineering process, it is an important technique for controlling surface texture on the face of clubs to increase the spin rate of the golf ball after impact, which is significantly affected by surface roughness and texture.7
Golf ball spin
A significant factor for golf ball spin is the surface roughness of the clubface. Several manufacturers have successfully marketed wedge clubs with a higher surface roughness than a standard club in an effort to create high initial spin rates. As with the geometry of the grooves, the surface roughness is regulated by the USGA, with a current limit of Ra 180 m in (0.045 mm). While previous efforts to create high spin faces focused on merely increasing the surface roughness, laser surface modi cation provides an opportunity to orient the surface texture to increase the level of backspin without increasing the level of deleterious sidespin imparted to the ball.
Golf balls sitting on a putting green rest in a small depression when they are stationary. In order to get the ball rolling, it must be lifted slightly up to the surface of the green. Most putter designs account for this by providing a small amount of loft (typically between 2–5°) to get the ball up onto a rolling plane. There is, however, a negative consequence of the putter’s loft. During impact, the ball slides up the putter face producing a very small amount of backspin allowing the ball to skid along the grass. During this ‘skid zone’ the ball can easily travel off the intended line due to the uneven texture of the green. The skid zone is typically 15–30 in (~38–76 cm).
A textured putter face allows the ball to be ‘grabbed’ which prevents it from sliding up the face on impact, minimizing the skidding and producing a more solid roll (Fig. 1). Several companies have successfully introduced putters with CNC milled grooves to minimize skidding and have received positive acceptance in the marketplace.
Many advances have been made in optimizing the feel of golf clubs through the use of composite graphite shafts, advanced polymeric grips and viscoelastic materials placed on the back of the hitting face of clubs to dampen vibration.10 Recent research has shown that surface texture can also enhance feel by transmitting the vibrations away from the impact zone on the club face, preventing excitation of the high-frequency vibrational bending modes which cause unpleasant feel. Figure 2 shows a laser profilometer scan of the surface of a driver golf club face that has been treated with laser radiation. The individual laser pulses and vibration channels are evident.
1.SEM micrograph of ultrafine laser features
2. Laser profilometer scan of Ti – 6Al – 4V golf
For many years, golf club manufacturers have recognized the importance of golf club face design to the overall performance of the club. The primary focus has been the material composition, surface roughness and the shape of the grooves cast or milled into the surface. More recently, interesting opportunities have arisen in the field of surface engineering, providing fine-scale textures that have a significant impact on the performance of golf clubs. Laser surface modification is of particular interest due to its low cost, scalability, and compatibility with the common materials used in golf club construction. Given the increasing level of competition among golf club manufacturers, new advanced materials technologies are likely to enter the market and differentiate between the performance and playability of clubs.
Dr Darin Aldrich President and CEO Photon Golf, Inc.