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Arc evaporated wear-resistant nitride coatings for metal cutting tools
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2008 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This Thesis is dedicated to increase the understanding of arc evaporated PVD coatings as wear resistant layers on metal cutting tools. The approach is to study coatings that have excellent performance in metal cutting applications, specifically (Ti,Al)N and (Ti,Si)N in terms of thermal and mechanical properties, and to correlate this to their microstructure, stress state, and composition. The effect of addition of oxygen into (Ti,Al)N is also evaluated in terms of structure, chemical bonding, and mechanical properties. It is shown that metastable fcc-(Ti,Al)N coatings separate into Ti-rich and Al-rich fcc-(Ti,Al)N zones via spinodal decomposition at 800 - 1000 °C, which acts as a hardening mechanism. This is followed by nucleation and growth into the stable phases fcc-TiN and hex-AlN at T>1000°C, with subsequent loss of hardness. These structural changes are correlated to the cutting performance, showing that the initial spinodal phase separation improves the performance. The success of (Ti,Al)N in metal cutting applications is, hence, due not only to the well documented oxidation resistance, but also to the spinodal decomposition process, which is active at the typical temperatures at the cutting edge of an engaged cutting insert. The potential subsequent renucleation process is, however, detrimental in metal cutting applications. Oxygen is commonly regarded as a contamination in PVD coating processes due to the risk of formation of insulating layers. This study, however, shows that by using arcevaporation, up to 35 at.% O can be incorporated into (Ti,Al)N coatings without altering its NaCl-structure. 1t is inferred that O substitutes for N in the lattice and (Ti,Al)(O,N) is formed. The incorporation of small amounts of oxygen (up to 13 at.%) improves the cutting performance by reducing the risk of chipping. However, at higher oxygen levels, the wear resistant properties are dramatically reduced. Finally, is shown that it is poss ible to replace at least 14 at.% Ti by Si, without altering the NaCl-structure in (Ti,Si)N coatings. The measured hardness of solid solution fcc-(Ti,Si)N is nearly a linear function of Si-content in the coating (from 31 GPa in TiN up to 45 GPa in (Ti0.86Si0.14)N). The hardness is also retained after annealing at 900 oC for 2h.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2008. , 33 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1375
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-44911Local ID: 78251ISBN: 978-91-7393-849-5 (print)OAI: oai:DiVA.org:liu-44911DiVA: diva2:265773
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2013-11-22
List of papers
1. Thermal stability of arc evaporated high aluminum-content Ti1−xAlxN thin films
Open this publication in new window or tab >>Thermal stability of arc evaporated high aluminum-content Ti1−xAlxN thin films
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2002 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 20, no 5, 1815-1823 p.Article in journal (Refereed) Published
Abstract [en]

The thermal stability of Ti1−xAlxN films deposited by arc evaporation from Ti–Al cathodes with 67 and 75 at. % aluminum, respectively, has been investigated. The microstructure of as-deposited and isothermally annealed samples were studied using scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The chemical composition and elemental distribution were determined by energy dispersive x ray (EDX), Rutherford backscattering spectrometry, and EDX mapping. Transmission electron micrographs revealed a dense and columnar microstructure in the as-deposited condition. Films deposited from the 67 at. % cathodes were of cubic NaCl-structure phase, whereas films deposited from the 75 at. % cathodes exhibited nanocrystallites of wurzite-structure hexagonal-phase AlN in a cubic (c)-(Ti,Al)N matrix. Both films were stable during annealing at 900 °C/120 min with respect to phase composition and grain size. Annealing at 1100 °C of films deposited from the 67 at. % cathodes resulted in phase separation of c-TiN and h-AlN, via spinodal decomposition of c-TiN and c-AlN. (Ti,Al)N films undergo extensive stress relaxation and defect annihilation at relatively high temperatures, and aspects of these microstructural transformations are discussed.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-18801 (URN)10.1116/1.1503784 (DOI)
Available from: 2009-06-08 Created: 2009-06-04 Last updated: 2017-12-13Bibliographically approved
2. Self-organized nanostructures in the Ti-Al-N system
Open this publication in new window or tab >>Self-organized nanostructures in the Ti-Al-N system
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2003 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 83, no 10, 2049-2051 p.Article in journal (Refereed) Published
Abstract [en]

The phenomenon of age hardening could be evidenced in thin film applications. A model system, Ti1-xAlxN was chosen as such coatings are known for their excellent wear resistance enabling improved machining processes like high-speed and dry cutting. Here, we show unambiguously that metastable Ti1-xAlxN coatings initially undergo spinodal decomposition into coherent cubic-phase nanometer-size domains, causing an increase in hardness at elevated temperatures. These intermediate metastable domains transform into their stable phases TiN and AlN during further thermal treatment. Activation energies for the processes indicate defect-assisted segregation of Ti and Al. The findings are corroborated by ab initio calculations. A long-standing discussion on the thermal stability of this important class of ceramics is thus resolved.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-18822 (URN)10.1063/1.1608464 (DOI)
Available from: 2009-06-08 Created: 2009-06-05 Last updated: 2017-12-13Bibliographically approved
3. Mechanical properties and machining performance of Ti1−xAlxN-coated cutting tools
Open this publication in new window or tab >>Mechanical properties and machining performance of Ti1−xAlxN-coated cutting tools
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2005 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 191, no 2-3, 384-392 p.Article in journal (Refereed) Published
Abstract [en]

The mechanical properties and machining performance of Ti1−xAlxN-coated cutting tools have been investigated. Processing by arc evaporation using cathodes with a range of compositions was performed to obtain coatings with compositions x=0, x=0.25, x=0.33, x=0.50, x=0.66 and x=0.74. As-deposited coatings with x≤0.66 had metastable cubic structures, whereas x=0.74 yielded two-phase coatings consisting of cubic and hexagonal structures. The as-deposited and isothermally annealed coatings were characterised by nanoindentation, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Cutting tests revealing tool wear mechanisms were also performed. Results show that the Al content, x, promotes a (200) preferred crystallographic orientation and has a large influence on the hardness of as-deposited coatings. The high hardness (∼37 GPa) and texture of the as-deposited Ti1−xAlxN coatings are retained for annealing temperatures up to 950 °C, which indicates a superior stability of this system compared to TiN and Ti(C,N) coatings. We propose that competing mechanisms are responsible for the effectively constant hardness: softening by residual stress relaxation through lattice defect annihilation is balanced by hardening from formation of a coherent nanocomposite structure of c-TiN and c-AlN domains by spinodal decomposition. This example of secondary-phase transformation (age-) hardening is proposed as a new route for advanced surface engineering, and for the development of future generation hard coatings.

Keyword
Hard coatings; TiAlN; Age hardening; Spinodal decomposition; Transition; Metal nitrides
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-18823 (URN)10.1016/j.surfcoat.2004.04.056 (DOI)
Available from: 2009-06-08 Created: 2009-06-05 Last updated: 2017-12-13Bibliographically approved
4. Structure and mechanical properties of arc evaporated Ti–Al–O–N thin films
Open this publication in new window or tab >>Structure and mechanical properties of arc evaporated Ti–Al–O–N thin films
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2007 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 201, no 14, 6392-6403 p.Article in journal (Refereed) Published
Abstract [en]

The structure, mechanical properties, and machining performance of arc evaporated Ti–Al–O–N coatings have been investigated for an Al0.66Ti0.34 target composition and O2/(O2+N2) gas flow-ratio varied between 0 to 24%. The coating structure was analysed using SEM, EDX, XRD, XPS, TEM, and STEM. Mechanical properties were analysed using nanoindentation and the deformation behaviour was analysed by probing the nanoindentation craters. The coatings performances in cutting tests were evaluated in a turning application in low carbon steel (DIN Ck45). It is shown that the addition of oxygen into the arc deposition process leads to the formation of a dual layer structure. It consists of an initial cubic NaCl-structure solid solution phase formed closest to the substrate, containing up to 35 at.% oxygen (O/O+N), followed by steady-state growth of a nanocomposite compound layer comprised of Al2O3, AlN, TiN, and Ti(O,N). The addition of oxygen increases the ductility of the coatings, which improves the performances in cutting tests. At high levels of oxygen, (>13 at.%), however, the performance is dramaticallyreduced as a result of increased crater wear.

Keyword
TiAlON; Arc-evaporation; Nanostructure; Mechanical properties
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-18825 (URN)10.1016/j.surfcoat.2006.12.006 (DOI)
Available from: 2009-06-08 Created: 2009-06-05 Last updated: 2017-12-13Bibliographically approved
5. Influence of Si on the Microstructure of Arc Evaporated (Ti,Si)N Thin Films: Evidence for Cubic Solid Solutions and their Thermal Stability
Open this publication in new window or tab >>Influence of Si on the Microstructure of Arc Evaporated (Ti,Si)N Thin Films: Evidence for Cubic Solid Solutions and their Thermal Stability
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2005 (English)In: Surface and Coatings Technology, ISSN 0257-8972, Vol. 200, no 5-6, 1535-1542 p.Article in journal (Refereed) Published
Abstract [en]

Ti1−xSixN (0 ≤ x ≤ 0.14) thin solid films were deposited onto cemented carbide (WC-Co) substrates by arc evaporation. X-ray diffraction and transmission electron microscopy showed that all films were of NaCl-structure type phase. The as-deposited films exhibited a competitive columnar growth mode where the structure transits to a feather-like nanostructure with increasing Si content. Films with 0 ≤ x ≤ 0.01 had a 111 crystallographic preferred orientation which changed to an exclusive 200 texture for 0.05 ≤ x ≤ 0.14. X-ray photoelectron spectroscopy revealed the presence of Si–N bonding, but no amorphous Si3N4. Band structure calculations performed using a full potential linear muffin tin orbital method showed that for a given NaCl-structure Ti1−xSixN solid solution, a phase separation into cubic SiN and TiN is energetically favorable. The microstructure was maintained for the Ti0.86Si0.14N film annealed at 900 °C, while recrystallization in the cubic state took place at 1100 °C annealing during 2 h. The Si content influenced the film hardness close to linearly, by combination of solid-solution hardening in the cubic state and defect hardening. For x = 0 and x = 0.14, nanoindentation gave a hardness of 31.3 ± 1.3 GPa and 44.7 ± 1.9 GPa, respectively. The hardness was retained after annealing at 900 °C, while it decreased to below 30 GPa for 1100 °C following recrystallization and W and Co interdiffusion.

Keyword
Nitrides; Arc evaporation; Transmission electron microscopy (TEM); Thin films; Solid solution; Microstructure
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14144 (URN)10.1016/j.surfcoat.2005.08.096 (DOI)
Available from: 2006-11-15 Created: 2006-11-15 Last updated: 2016-08-31

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