Machine Design

Barely scratching the engineered surface: Extending part life with micron-scale surface treatments

Emerging engineered-surface treatments work at the micron scale to boost load capacity of bearings, gears, and other components.

A current challenge for automotive engineers is to raise drivetrain loads without upsizing parts.

The conventional approach first optimizes gear-tooth geometry, materials, and surface finishes of ground and honed parts. But after exhausting these "macro" techniques you may want to give surface engineering a look before initiating costly redesigns.

Timken Engineered Surfaces, for instance, alter the physical or chemical properties of component surfaces at the micron scale. Doing so can improve wear, fatigue, and frictional performance of bearings, hydrostatics, engine components, and gear trains. For example, engineering the surfaces of transmission gear teeth can extend life about threefold. Engineered Surfaces include topographical modification and coatings.

Topographical modification brings surface roughness below levels possible by conventional grinding or honing. The proprietary process lowers asperity heights, producing a flatter, nondirectional surface that is less prone to microwelding and adhesive wear than ground surfaces. A typical ground surface has about a 12 to 20 μin. finish. Topographical-modified surfaces, for comparison, have 2 to 3 μin. surface finishes.

Smoother surface finishes also raise the dimensionless quantity:

where, d = clearance between parts (in.) ands σ1 and σ2 are the surface finishes of mating parts (in.). Metal-to-metal contact happens at l, while lubricant separates surfaces forl<1. Smoother surfaces increase clearances and lowers sol goes up, which is desirable.

Certain topographical treatments also add residual compressive surface stresses that boost load capacity and fatigue resistance. For example, valve springs used to be the weak link in 700-hp Nascar racing engines. But surface modification now decreases the number and size of surface defects and fatigue initiation sites, greatly extending spring life.

For gear teeth, bending fatigue is the primary failure mode. Topographical treatments that impart compressive stresses tend to spread applied stresses along a tooth surface and inhibit the propagation of microcracks. At extremely high power densities and conditions of inadequate lubrication, applied stresses can cause microwelding of contact surfaces that in turn leads to pitting, spalling, and false brinelling. The resulting surface imperfections and ejected wear debris locally raise contact stresses that can trigger fatigue failures. But the application of a lubricious coating to one of a pair of topographicaltreated contact surfaces helps prevent microwelding and the related problems over a wide range of operating conditions.

Engineered Surface coatings consist of a thin layer of nanocrystalline metal carbides embedded in an amorphous, flexible, diamondlike matrix. The coatings are extremely hard — much harder than the underlying substrate — yet sufficiently elastic to flex with underlying substrates. And they won't lose adhesion, flake, or crack off. Their dissimilar material and hard surface discourages adhesive and abrasive wear on gear flanks under conditions of low speed and poor lubrication, raising pittingtorque limits by 33% in some cases. These same properties boost scuffing-torque limits by about 70% when high pitch-line velocities fling lubricant out of tooth-contact areas. In tapered-roller bearings, surface-finish modification and coatings reduce scuffing and scoring of roller ends and rib faces caused by inadequate lubrication.

Engineered surface coatings are applied under high vacuum by a physical vapordeposition (PVD) process similar to that used for the coating of silicon chips and tools, though the equipment and coating chemistries in this case are specific to tribological applications. Process temperatures rarely exceed 180°C so substrate hardness is unaffected. In contrast, high process temperatures characterizing other techniques can change surface tempering, which can hurt performance and shorten component life.

Coating thickness is about 1 to 3 μm and is dictated by tribological demands. A valve for controlling fluid delivery in high-pressure diesel-engine injectors needs a 1-μm-thick coating, while a roller in a steel mill may use a 3-μm coating, for example. Coatings are also tailored to the application and use different process conditions.

Components typified by rolling contact such as bearing races generally use ES320 coating. ES220 coating is better for sliding-contact elements including seals and tapered roller bearing ribs. Engineered Surface technology is applied during component manufacturing just prior to final assembly. Therefore, be sure to hold tolerance on previous manufacturing steps to avoid tolerance stack-up issues. Surface engineering can't make up for poor substrates, improper geometry, and profiles. Moreover, surface engineering isn't a one-size-fits-all approach, and new applications require thorough analysis and testing. In any case, consider topographical modifications first because they generally cost less than coatings.

MAKE CONTACT: (330) 438-3000 | Timken Co.

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