A hydraulic pump part fails at its running surface long before its bulk material gives up, which is why almost every part in a pump is a composite of a tough core and a hard or bearing-grade face. Cylinder blocks are steel bodies with a sintered bronze running face bonded and machined after sintering, then lapped flat. The bronze carries the sliding contact; the steel carries the load.

Pistons and shoes are case hardened and ground, with the shoe face hydrostatically balanced so the oil film rather than the metal takes the load. Valve plates and swash plates are through hardened and lapped, because they see both sliding contact and the pressure transition at every revolution.

Vanes run in a different regime again: they are thin, they slide radially at high frequency, and they are the part most sensitive to contamination. They are therefore hardened to a higher surface hardness than the cam ring they run against, so that when abrasive wear does happen the cheaper part loses.

Shafts are hardened only where they need to be. The spline and the seal journal are treated; the body is left tough so the shaft bends rather than snaps under shock loading.