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Tribology at the Cutting Edge: A Study of Material Transfer and Damage Mechanisms in Metal Cutting
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterials group)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The vision of this thesis is to improve the metal cutting process, with emphasis on the cutting tool, to enable stable and economical industrial production while using expensive tools such as hobs. The aim is to increase the tribological understanding of the mechanisms operating at a cutting edge and of how these can be controlled using different tool parameters. Such understanding will facilitate the development and implementation of future, tribologically designed, cutting tools.

Common wear and failure mechanisms in gear hobbing have been identified and focused studies of the material transferred to the tool, in both metal cutting operations and in simplified tribological tests, have been conducted. Interactions between residual stresses in the tool coating and the shape of the cutting edge have also been studied.

It was concluded that tool failure is often initiated via small defects in the coated tool system, and it is necessary to eliminate, or minimize, these defects in order to manufacture more reliable and efficient gear cutting tools. Furthermore, the geometry of a cutting edge should be optimized with the residual stress state in the coating, in mind. The interaction between a compressive stress and the geometry of the cutting edge will affect the stress state at the cutting edge and thus affect the practical toughness and the wear resistance of the coating in that area.

An intermittent sliding contact test is presented and shown to be of high relevance for studying the interaction between the tool rake face and the chip in milling. It was also demonstrated that material transfer, that can have large effects on the cutting performance, commences already after very short contact times. The nature of the transfer may differ in different areas on the tool. It may include glassy layers, with accumulations of specific elements from the workpiece, and transfer of steel in more or less oxidized form. Both tool coating material, its surface roughness, and the relative speed between the tool surface and the chip, may influence the extent to which the different transfer will occur.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 77 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 988
Keyword [en]
Tribology, Metal cutting, Gear hobbing, Wear, Coating, Residual stress, Material transfer, Steel
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
URN: urn:nbn:se:uu:diva-183186ISBN: 978-91-554-8514-6 (print)OAI: oai:DiVA.org:uu-183186DiVA: diva2:562353
Public defence
2012-12-07, 2005, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (Swedish)
Supervisors
Available from: 2012-11-16 Created: 2012-10-23 Last updated: 2013-01-23Bibliographically approved
List of papers
1. Survey of damage mechanisms on PVD coated HSS hobs used in Swedish gear manufacturing industry
Open this publication in new window or tab >>Survey of damage mechanisms on PVD coated HSS hobs used in Swedish gear manufacturing industry
2011 (English)In: Tribologia, ISSN 0780-2285, Vol. 30, no 1-2, 37-50 p.Article in journal (Refereed) Published
Abstract [en]

Gear hobbing is widely used for production of cylindrical gears in the Swedish transmission industry. The hob, usually consisting of a homogenous HSS (High Speed Steel) body coated with a ceramic PVD (Physical Vapour Deposition) coating, is designed for regrinding and recoating several times without affecting its cutting geometries. Efficient usage of the tool, considering production costs and gear quality, requires reconditioning before wear starts to affect the gear quality negatively and certainly before tool wear renders reconditioning impossible. Hobs of today generally lack in reliability, making it difficult to judge when they have to be taken out for reconditioning.

 

This work presents a survey of wear as observed on today’s state of the art hobs used by Swedish gear manufactures. It aims to identify damage mechanisms and the common problems in order to enable future production of more reliable hobs. The tools were temporarily borrowed from the production and the analyses were made non-destructively using optical microscopes. This was complemented by destructive cross-sectional analysis on two of the hobs.

 

Wear was most commonly located on the rake faces and the cutting edges of the cutting teeth. It mainly propagates by discrete fractures which appear to originate at local defects in the coating or at the interface. High intrinsic stress in the coating likely promotes coating spallation and accelerates the wear of the cutting edge.

Keyword
gear hobbing, hob, wear, HSS, cutting tool, PVD
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-165817 (URN)
Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2016-04-19Bibliographically approved
2. On the wear of PVD-coated HSS hobs in dry gear cutting
Open this publication in new window or tab >>On the wear of PVD-coated HSS hobs in dry gear cutting
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2009 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 266, no 3-4, 444-452 p.Article in journal (Refereed) Published
Abstract [en]

A hob is an advanced gear cutting tool usually made of homogenous high-speed steel (HSS) and coated by physical vapour deposition (PVD). It is designed for regrinding and recoating many times. However, hobs of today suffer from unpredictable wear making it difficult to schedule when they should be taken out for reconditioning. This investigation is aimed to contribute to the fundamental knowledge of the wear mechanisms and wear propagation of hobs used in dry gear cutting. Two AlCrN-coated hobs, used in actual gear cutting, were investigated by scanning electron microscopy (SEM) to study the tool topography, light optical microscopy (LOM) to study metallographic cross-sections and by X-ray photoelectron spectroscopy (XPS) for surface analyses.

It is concluded that the high potential of PVD-coated HSS as tool material is not taken full advantage of. The main reason is a combination of a poor surface preparation prior to coating and excessively high compressive residual stresses, making parts of the coating detach along the edge line even on unused cutting teeth. During cutting, the high intrinsic stresses in combination with droplets and defects in the coating facilitated its fragmentation through decohesion and detachment. Simultaneously, the rake face is gradually covered by a thin oxide layer dominated by Fe, Cr and Mn in about equal amounts, and also with a significant content of Si. It is believed but not proved that this layer is beneficial.

To improve service life and reliability of gear cutting hobs, precautions should be taken for the grinding–polishing preparation prior to PVD-coating to ensure a smooth substrate free from burning damage and other defects. Also, the edge radius and coating thickness has to be matched with the magnitude of compressive residual stresses in the coating.

Keyword
Gear hobbing, High-speed steel, PVD-coating, Wear propagation
National Category
Materials Engineering
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-111756 (URN)10.1016/j.wear.2008.04.014 (DOI)000263217300011 ()
Projects
KUGG
Available from: 2009-12-21 Created: 2009-12-21 Last updated: 2017-12-12
3. Evaluation of an intermittent sliding test for reproducing work material transfer in milling operations
Open this publication in new window or tab >>Evaluation of an intermittent sliding test for reproducing work material transfer in milling operations
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2012 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 52, 153-160 p.Article in journal (Refereed) Published
Abstract [en]

TiN coated HSS test cylinders from an intermittent sliding test were compared with TiN coated HSS milling inserts from a single insert milling test. A 20NiCrMo2 case hardening steel was used as counter material and work material in the two tests. HSS test cylinders, coated with AlCrN, TiAlN and Al2O3, were also tested in the intermittent sliding test and evaluated with regard to material adhesion. Two distinctly different tribofilms were formed on test cylinders as well as on cutting inserts. One consisting of Mn, Si, Al and O and one consisting of Fe, Mn, Cr, Si and O. The transferred material was similar in appearance and composition, both when comparing the two test methods and when comparing the different tool coatings.

Keyword
Transfer, Coating, Chip, Sliding
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Tribo Materials; Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-167051 (URN)10.1016/j.triboint.2012.03.015 (DOI)000304798900015 ()
Available from: 2012-01-19 Created: 2012-01-19 Last updated: 2017-12-08Bibliographically approved
4. Influence of sliding speed on modes of material transfer as steel slides against PVD tool coatings
Open this publication in new window or tab >>Influence of sliding speed on modes of material transfer as steel slides against PVD tool coatings
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2013 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 58, 55-64 p.Article in journal (Refereed) Published
Abstract [en]

An intermittent sliding test was used in order to study the formation and build-up of tribofilms during intermittent sliding of PVD coated HSS against case hardening steel (20NiCrMo2). Two cutting tool coatings were tested, TIN and AlCrN, and the influence of sliding speed was evaluated. With moderate speed, two tribofilms were formed separately, one consisting of Mn, Si, Al and O on an intermediate layer of Fe and one consisting of Fe, Mn, Cr and O on an intermediate layer of Cr and Mn. At low sliding speeds an uneven transfer of steel occured while high sliding speeds resulted in thermal softening of the substrate leading to coating failure. AlCrN provided better substrate protection at high speeds than TiN did.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-167053 (URN)10.1016/j.triboint.2012.09.012 (DOI)000312610600008 ()
Available from: 2012-01-19 Created: 2012-01-19 Last updated: 2017-12-08Bibliographically approved
5. Influence from surface roughness on steel transfer to PVD tool coatings in continuous and intermittent sliding contacts
Open this publication in new window or tab >>Influence from surface roughness on steel transfer to PVD tool coatings in continuous and intermittent sliding contacts
Show others...
2012 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 56, 9-18 p.Article in journal (Refereed) Published
Abstract [en]

A sliding test is used in order to evaluate the influence of tool surface roughness on the material transfer in intermittent and continuous sliding of PVD coated HSS against case hardening steel (20NiCrMo2). Two cutting tool coatings, TiN and AlCrN, and three different surface roughnesses are tested. For polished surfaces the same types of material transfer are obtained irrespective of sliding mode and coating type. If the surfaces are too rough, the tribofilms do not grow thick enough to separate the surfaces and the work material is abrasively worn in both sliding modes. With increased sliding distance, cracking of the TiN coating occurs while the AlCrN coating remains intact.

Keyword
Transfer, coating, sliding, roughness
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-167054 (URN)10.1016/j.triboint.2012.06.013 (DOI)000308284000002 ()
Funder
VINNOVA
Available from: 2012-01-19 Created: 2012-01-19 Last updated: 2017-12-08Bibliographically approved
6. Adhesion phenomena in the secondary shear zone in turning of austenitic stainless steel and carbon steel
Open this publication in new window or tab >>Adhesion phenomena in the secondary shear zone in turning of austenitic stainless steel and carbon steel
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2014 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 214, no 8, 1467-1481 p.Article in journal (Refereed) Published
Abstract [en]

This paper aims to increase the understanding of the adhesion between chip and tool rake face by studying the initial material transfer to the tool during orthogonal machining at 150 m/min. Two types of work material were tested, an austenitic stainless steel, 316L, and a carbon steel, UHB 11. The tools used were cemented carbide inserts coated with hard ceramic coatings. Two different CVD coatings, TiN and Al2O3, produced with two different surface roughnesses, polished and rough, were tested. The influences of both tool surface topography and chemistry on the adhesion phenomena in the secondary shear zone were thus evaluated. Extensive surface analyses of the inserts after cutting were made using techniques such as Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM). As expected, cutting in the stainless steel resulted in a higher amount of adhered material, compared to cutting in the carbon steel. Remnants of built-up layers were found on the surfaces of the 316L chips but not on the UHB 11 chips. Moreover, it was shown that for both materials the tool roughness had a profound effect, with the rougher surfaces comprising much higher amounts of adhered material than the polished ones. Non-metallic inclusions from both types of workpiece steels accumulate in the high temperature area on the inserts. The general tendency was that higher amounts of transferred material were found on the TiN coating than on the Al2O3 coating after cutting.

Keyword
adhesion, austenitic stainless steel, carbon steel, surface roughness, cutting tools
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-183185 (URN)10.1016/j.jmatprotec.2014.01.017 (DOI)000336341300001 ()
Available from: 2012-10-23 Created: 2012-10-23 Last updated: 2017-12-07Bibliographically approved
7. Assessing the hardness and residual stress at the very edge of a TiAlN coated cutting insert
Open this publication in new window or tab >>Assessing the hardness and residual stress at the very edge of a TiAlN coated cutting insert
Show others...
(English)In: Surface and coatings technology, ISSN 0257-8972Article in journal (Refereed) Submitted
Abstract [en]

A method for determining residual stresses through nanoindentation has been used in order to evaluate the local residual stress in a PVD Ti0.5Al0.5N coating. The influence of residual stress on the measured hardness is obtained by comparing the hardness of as deposited, residually stressed, coating and stress relieved pillars made in the very same coating. The technique offers high lateral resolution, and it can be applied on complex geometries where conventional stress measurements fall short. In this work, the method proved useful for estimating the residual stress at such a complex geometry as a sharp cutting edge. Edges with two different radii were analysed and it was concluded that the nominal residual compressive stress given to a coating can locally, on the very cutting edge, become significantly reduced by elastic relaxation .This effects the cohesion of the coating at that position which, together with the lift-off stress generated at the edge, make the coating in that region especially vulnerable to damage. The experimental results correlated very well with finite element simulations of the residual stress state around cutting edges with the same geometries.

Place, publisher, year, edition, pages
Elsevier
Keyword
Residual stress, PVD coating, Cutting edge, Nanoindentation
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-183161 (URN)
Available from: 2012-10-23 Created: 2012-10-23 Last updated: 2013-01-23Bibliographically approved

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