TY - BOOK

T1 - Synthesis-Structure-Property Relations for Arc Evaporated Al-Cr-O Hard Coatings

AU - Pohler, Markus

N1 - embargoed until 18-04-2019

PY - 2018

Y1 - 2018

N2 - Synthesis of corundum type Al2O3 coatings, which are applied for instance as wear protection in metal cutting, is well-established and typically based on the chemical vapour deposition technology. However, due to deposition temperatures above 800 °C their application is restricted to heat resistant substrate materials like cemented carbides and requires reliable process control. Attempts to transfer the Al2O3 synthesis to physical vapour deposition processes, using deposition temperatures below 600 °C, has proven to be difficult. Alternatively, (AlxCr1-x)2O3 solid solution coatings gained high scientific and industrial attention as Cr2O3, which is isostructural to corundum type Al2O3 with similar lattice parameters, promotes the corundum growth within the desired temperature range. In this work, reactive cathodic arc evaporated (AlxCr1-x)2O3 coatings grown from Al-Cr compound cathodes are discussed with respect to their phase composition, thermal stability as well as mechanical and tribological properties. Based on a systematic variation of synthesis conditions, comprising O2 flow rate as well as substrate bias parameters and the Al/(Al+Cr) ratio within the cathode from 0.25 to 0.85, a basic understanding on the interaction between deposition, structure and properties was established. First, the work focuses on the cathode processes and their impact on coating morphology and microstructure. This resulted in a detailed description of the remelted zone formed during evaporation on the cathode surface and its effect on macroparticles incorporated into the growing coating. In a second step, the limits of corundum type coating growth were illuminated by stepwise increasing the Al2O3 mole fraction within the coating. A thermally stable corundum type structure was established up to an Al2O3 content of ~50 mol%. Beyond, multiphase coating structures with a mixture of stable and metastable components arose. The detected metastable transient structures transformed to the stable corundum configuration during post deposition annealing at ~1000 °C. In a third step, the effect of a crystallographic template in a bi-layer coating architecture on the microstructure was investigated, aiming to stimulate corundum type growth in a high Al2O3 containing (AlxCr1-x)2O3 coating. Finally, the characteristics of the applied bipolar-pulsed substrate bias were modified by increasing the bias potential amplitude from 40 to 160 V and by varying the ratio between negative and positive pulse duration, i.e. the duty cycle. Thereby, increasing the bias amplitude yielded decreased crystallite sizes and a pronounced coating texture, while the duty cycle affected residual coating stresses, domain sizes and mechanical properties.

AB - Synthesis of corundum type Al2O3 coatings, which are applied for instance as wear protection in metal cutting, is well-established and typically based on the chemical vapour deposition technology. However, due to deposition temperatures above 800 °C their application is restricted to heat resistant substrate materials like cemented carbides and requires reliable process control. Attempts to transfer the Al2O3 synthesis to physical vapour deposition processes, using deposition temperatures below 600 °C, has proven to be difficult. Alternatively, (AlxCr1-x)2O3 solid solution coatings gained high scientific and industrial attention as Cr2O3, which is isostructural to corundum type Al2O3 with similar lattice parameters, promotes the corundum growth within the desired temperature range. In this work, reactive cathodic arc evaporated (AlxCr1-x)2O3 coatings grown from Al-Cr compound cathodes are discussed with respect to their phase composition, thermal stability as well as mechanical and tribological properties. Based on a systematic variation of synthesis conditions, comprising O2 flow rate as well as substrate bias parameters and the Al/(Al+Cr) ratio within the cathode from 0.25 to 0.85, a basic understanding on the interaction between deposition, structure and properties was established. First, the work focuses on the cathode processes and their impact on coating morphology and microstructure. This resulted in a detailed description of the remelted zone formed during evaporation on the cathode surface and its effect on macroparticles incorporated into the growing coating. In a second step, the limits of corundum type coating growth were illuminated by stepwise increasing the Al2O3 mole fraction within the coating. A thermally stable corundum type structure was established up to an Al2O3 content of ~50 mol%. Beyond, multiphase coating structures with a mixture of stable and metastable components arose. The detected metastable transient structures transformed to the stable corundum configuration during post deposition annealing at ~1000 °C. In a third step, the effect of a crystallographic template in a bi-layer coating architecture on the microstructure was investigated, aiming to stimulate corundum type growth in a high Al2O3 containing (AlxCr1-x)2O3 coating. Finally, the characteristics of the applied bipolar-pulsed substrate bias were modified by increasing the bias potential amplitude from 40 to 160 V and by varying the ratio between negative and positive pulse duration, i.e. the duty cycle. Thereby, increasing the bias amplitude yielded decreased crystallite sizes and a pronounced coating texture, while the duty cycle affected residual coating stresses, domain sizes and mechanical properties.

KW - Lichtbogenverdampfung

KW - (Al

KW - Cr)2O3

KW - Verbundkathode

KW - Schichtwachstum

KW - Template-Effekt

KW - Mikrostruktur

KW - Gepulstes Bias

KW - arc evaporation

KW - (Al

KW - Cr)2O3

KW - compound cathode

KW - coating growth

KW - template effekt

KW - microstructure

KW - pulsed bias

M3 - Doctoral Thesis

ER -