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Surface mechanical and thermochemical properties of materials are closely related to corrosion, fatigue, wear, wettability,

biocompatibility of the material and thus are critically important in the reliability, cost effectiveness, applicability, or safety of machinery

systems and devices. Accordingly, effective and reliable surface treatment techniques of materials are of great interest of industry and

engineering.

Laser surface treatment is a solution for many of those needs and often it is the only solution for a problem. Laser can raise the

temperature on the irradiated surface to several thousands of degree and pressure to hundreds of atmosphere in less than a few

nanoseconds so that heating, melting, or the generation of intense compressive stress of the material can be achieved easily. By

properly controlling process parameters, the thickness of altered layer, level of temperature and pressure, area of processing, etc. can

be control with great accuracy. Furthermore, since the laser beam strikes local areas only, unwanted thermal effects such as large heataffected zone can be avoided with laser surface treatment.

Owing to the advantages of laser surface treatment, various types of laser surface treatment have been applied for metals,

semiconductors, or alloys. For example, laser transformation hardening of pearlitic grey cast iron using a kilowatt level continuous wave

Nd:YAG laser beam for automotive applications, laser shock hardening of aluminum alloys and titanium alloys for aerospace industry,

laser shock hardening of stainless steel for nuclear power plant, laser treatment of bone-implant materials to improve bio-compatibility,

laser alloying of magnesium alloys to enhance hardness and corrosion resistance, laser modification of surface properties of silicon to

improve its wettability and adhesion characteristics, laser cladding of turbine blades, etc. have been reported (FIG. 1).

(FIG. 1 Application examples of laser shock peening for surface treatment of metals)

In our laboratory, we study laser shock hardening, also known as laser shock peening, of duplex and AISI 304 stainless steels for the

application to high pressure and high capacity (HPHC) pump. HPHC pumps are installed in desalination plants or nuclear power plant

which is a highly corrosive environment. Enhancement of anti-corrosion and anti-wear properties of HPHC pumps for these facilities will

substantially save the risk of facility failure, maintenance cost, and the necessity of parts replacement.In laser shock peening, a high power Nd:YAG laser pulse irradiates the workpiece immersed in liquid, typically water, to generate high

pressure ablation plasma (FIG. 2). Since the ablation plasma is prevented from expansion by the confining liquid, it exerts a strong

compressive pressure on the workpiece, which in turn produces a strong compressive stress field in the medium near the irradiated

surface. Laser shock peening is in principle similar to the cold working, but produces much deeper stress field and smoother surface

than shot peening. Form experiments,we achieved 30-50% increase of hardness and over 50% increase of friction coefficient (FIG. 3).

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(FIG. 2 Photograph of laser shock peening of duplex stainless steel using 532 nm Nd:YAG laser)

(FIG. 3 Enhancement of surface hardness after laser shock peening of STS 304)