TY - THES

T1 - Damage initiation in hard-coated hard metal substrates under normal and tangential loads at elevated temperature

AU - Walch, Lukas

N1 - embargoed until null

PY - 2021

Y1 - 2021

N2 - In the manufacturing industry, WC-Co hard metals are among the most important tool materials. They are used for machining applications like the milling of high-strength materials. Their lifespan is often limited by defects in the hard metal substrate. Among the main causes for defect initiation and growth are the significant multi-axial loads and high temperatures present e.g. at the cutting edge of milling inserts. Presently there is a lack of testing methods capable of reproducing such conditions in controlled environments. In the current work a novel ball-in-cone test setup utilizes a spherical indenter and an inclined sample surface to introduce cyclic multi-axial loads into the specimen substrate in isothermal conditions. The setup is used to study the contact fatigue behavior of TiN-TiB2 hard-coated WC-12 wt.% Co hard metal specimens with a mean grain size of 2 µm. Cyclic loads of a level comparable to that seen at the cutting edge of milling tools were applied to tested specimens at a temperature of 700 °C induced via eddy current heating in a vacuum. The specimens’ microstructures were documented using scanning electron microscopy on sections exposed via focused ion beam milling. The stress situation in the specimen substrate was studied with a finite element simulation using an experimentally parameterized materials model also considering creep. The simulation showed stresses ranging from mainly compressive to tensile-compressive. Defect formation trends at up to 10,000 load cycles could be correlated with the applied stress ratio. Positions subjected to a combination of tensile and compressive stresses showed a significantly higher defect formation rate than positions with mainly compressive stresses. The defect initiation behavior in specimens under multi-axial loads at elevated temperature can be studied using the ball-in-cone test method.

AB - In the manufacturing industry, WC-Co hard metals are among the most important tool materials. They are used for machining applications like the milling of high-strength materials. Their lifespan is often limited by defects in the hard metal substrate. Among the main causes for defect initiation and growth are the significant multi-axial loads and high temperatures present e.g. at the cutting edge of milling inserts. Presently there is a lack of testing methods capable of reproducing such conditions in controlled environments. In the current work a novel ball-in-cone test setup utilizes a spherical indenter and an inclined sample surface to introduce cyclic multi-axial loads into the specimen substrate in isothermal conditions. The setup is used to study the contact fatigue behavior of TiN-TiB2 hard-coated WC-12 wt.% Co hard metal specimens with a mean grain size of 2 µm. Cyclic loads of a level comparable to that seen at the cutting edge of milling tools were applied to tested specimens at a temperature of 700 °C induced via eddy current heating in a vacuum. The specimens’ microstructures were documented using scanning electron microscopy on sections exposed via focused ion beam milling. The stress situation in the specimen substrate was studied with a finite element simulation using an experimentally parameterized materials model also considering creep. The simulation showed stresses ranging from mainly compressive to tensile-compressive. Defect formation trends at up to 10,000 load cycles could be correlated with the applied stress ratio. Positions subjected to a combination of tensile and compressive stresses showed a significantly higher defect formation rate than positions with mainly compressive stresses. The defect initiation behavior in specimens under multi-axial loads at elevated temperature can be studied using the ball-in-cone test method.

KW - WC-Co

KW - Hartmetall

KW - hartbeschichtet

KW - Ball-in-Cone

KW - Hochtemperatur

KW - Schädigungsinitiation

KW - Defektinitiation

KW - multiaxiale Lasten

KW - Tangentiallast

KW - Normallast

KW - WC-Co

KW - hard metal

KW - hard-coated

KW - Ball-in-Cone

KW - high temperature

KW - damage initiation

KW - defect initiation

KW - multi-axial loads

KW - tangential loads

KW - normal loads

M3 - Master's Thesis

ER -