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Investigation of topography, adhesion and diffusion in sliding contacts during steel and titanium alloy machining
Dalarna University, School of Technology and Business Studies, Materials Technology. Uppsala universitet. (Tribology)ORCID iD: 0000-0001-5536-3077
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of the present thesis work is to increase the fundamental knowledge of the tribological contact between the cutting tool and the work material in three different cutting operations, i.e. hard milling of cold work tool steels, turning in 316L stainless steel and turning in Ti6Al4V alloy, respectively. The influence of cutting parameters and tool surface topography on the initial material transfer tendency and resulting wear and wear mechanisms were investigated under well controlled cutting conditions. High resolution scanning electron microscopy (SEM) and surface analysis, including energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), were used in order to characterize the worn cutting tools on a sub-µm scale and deepen the understanding of the wear mechanisms prevailing at the tool / work material interface. The characterization work includes the analysis of worn tool surfaces as well as cross-sections of these. Also, the back side of collected chips were analysed to further understand the contact mechanisms between the tool rake face and chip.

The results show that the transfer tendency of work material is strongly affected by the surface topography of the rake face and that an appropriate pre- and post-coating treatment can be used in order to reduce the transfer tendency and the mechanical interaction between the mating surfaces. The continuous wear mechanisms of the cutting tools were found to be dependent on the work materials and the cutting parameters used. In hard milling of cold work tool steels, polycrystalline cubic boron nitride shows a combination of tribochemical wear, adhesive wear and mild abrasive wear. In the turning of 316L stainless steel and Ti6Al4V alloy, using medium to high cutting speeds/feeds, the wear of cemented carbide is mainly controlled by diffusion wear of the WC phase. Interestingly, the diffusion wear processes differ between the two work materials. In contact with 316L stainless steel crater wear is controlled by atomic diffusion of W and C into the passing chip. In contact with Ti6Al4V crater wear is controlled by the diffusion of C into a transfer work material layer generating a W-rich and TiC interfacial layer which repeatedly is removed by the passing chip. The experimental work and results obtained illustrates the importance of in-depth characterization of the worn surfaces in order to increase the understanding of the degradation and wear of tool materials and coatings in metal cutting operations.

Place, publisher, year, edition, pages
Uppsala University, 2019.
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
URN: urn:nbn:se:du-29700OAI: oai:DiVA.org:du-29700DiVA, id: diva2:1297437
Public defence
(English)
Supervisors
Available from: 2019-03-27 Created: 2019-03-19 Last updated: 2019-04-08Bibliographically approved
List of papers
1. Wear of a high cBN content PCBN cutting tool during hard milling of powder metallurgy cold work tool steels
Open this publication in new window or tab >>Wear of a high cBN content PCBN cutting tool during hard milling of powder metallurgy cold work tool steels
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2015 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 332, p. 752-761Article in journal (Other academic) Published
Abstract [en]

The wear characteristics of a high cBN content PCBN cutting tool during hard milling of two different hardened cold work tool steels have been evaluated. Post-cutting examination of the worn cutting inserts was performed using high resolution field emission gun scanning electron microscopy, energy dispersive X-ray spectroscopy, Auger electron spectroscopy and optical surface profilometry. Also, the machined work material surfaces and collected chips were characterized in order to evaluate the prevailing wear mechanisms. The results show that both flank and crater wear are controlled by continuous wear due to tribochemical reactions, adhesive wear and mild abrasive wear. Besides, the cutting inserts show a tendency to micro-chipping along the cutting edge especially at higher cutting speed. The latter mechanism was also found to be dependent on type of work material. High lateral resolution Auger electron spectroscopy of the crater region shows that the worn surface is covered by a thin SixOy rich tribofilm with a thickness of 50-500 nm, the tribofilm being thicker on the binder phase regions. Also, the Co-rich regions of the binder phase seem to be more tribochemically affected by the prevailing contact conditions as compared with the W-rich regions of the binder phase and the cBN phase. 

Keywords
PCBN; Milling; Cold work tool steel; Wear mechanisms; Surface analysis; AES
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-16352 (URN)10.1016/j.wear.2015.01.073 (DOI)000357438100014 ()
Available from: 2014-11-13 Created: 2014-11-13 Last updated: 2019-03-26Bibliographically approved
2. Influence of CVD and PVD coating micro topography on the initial material transfer of 316L stainless steel in sliding contacts: A laboratory study
Open this publication in new window or tab >>Influence of CVD and PVD coating micro topography on the initial material transfer of 316L stainless steel in sliding contacts: A laboratory study
2017 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 388-389, p. 29-38Article in journal (Refereed) Published
Abstract [en]

Austenitic stainless steels generally display poor tribological properties in sliding contacts partly due to their strong adhesion and transfer tendency to the counter surface. As a result machining of austenitic stainless steels is frequently associated with significant problems such as high stresses and high temperatures resulting in rapid tool wear. In the present study, the influence of coating micro topography on the initial material transfer of 316L stainless steel in sliding contacts has been evaluated using a scratch testing equipment. Coating materials include modern CVD Ti(C,N)-Al2O3-TiN and PVD (Ti,Al)N-(Al,Cr2)O3 coatings deposited on cemented carbide and pre- and post-coating grinding and polishing treatments were used to obtain different micro topographies of the coating surface. Pre- and post-test characterization of the surfaces was performed using high resolution scanning electron microscopy, energy dispersive X-ray spectroscopy and optical surface profilometry.

The results show that the intrinsic topography of the as-deposited CVD and PVD coatings promotes material transfer. For the as-deposited CVD coating the nanoscale topography of the crystals controls the transfer while for the PVD coating the µm-scale droplets and craters control the transfer. Post-polishing of the coating, especially in combination with pre-polishing of the substrate, significantly improves the tribological performance of the surface reducing the friction coefficient and the material transfer tendency. However, the presence of µm sized droplets and craters in the PVD coating limit the possibility to obtain a smooth post-polished surface and its resistance to material pick-up. In contrast, post-polishing of the CVD coating does not suffer from intrinsic coating defects which results in low friction and a very high resistance to material pick-up.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Material transfer; CVD and PVD coatings; Surface topography; Stainless steel; Metal cutting
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-23915 (URN)10.1016/j.wear.2016.12.003 (DOI)
Available from: 2017-01-10 Created: 2017-01-10 Last updated: 2019-03-26Bibliographically approved
3. Influence of tool surface topography on the material transfer tendency and tool wear in the turning of 316L stainless steel
Open this publication in new window or tab >>Influence of tool surface topography on the material transfer tendency and tool wear in the turning of 316L stainless steel
2016 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 368–369, p. 239-252Article in journal (Refereed) Published
Abstract [en]

Abstract The influence of tool surface topography on the initiation and build-up of transfer layers in the orthogonal turning of 316L austenitic stainless steel have been studied under well controlled conditions. Tool materials include CVD Ti(C,N)-Al2O3-TiN and PVD (Ti, Al)N-(Al,Cr)2O3 coated cemented carbide inserts prepared using different grinding and polishing treatments. Post-test characterization of the inserts was performed using high resolution scanning electron microscopy and energy dispersive X-ray spectroscopy. The results show that the transfer tendency of work material is strongly affected by the surface topography of the rake face. For both types of inserts, the initial transfer and the build-up of transfer layers are localised to microscopic surface irregularities on the rake face. Consequently, an appropriate surface treatment of the cemented carbide substrate before coating deposition and the as-deposited CVD and PVD coating can be used in order to reduce the transfer tendency and the mechanical interaction between the mating surfaces. Also, an improved surface finish was found to reduce coating wear and consequently the crater wear rate of the inserts investigated. This can most likely be explained by the reduced tendency to discrete chipping of coating fragments in the contact zone and the formation of a thin transfer layer composed of Al, Si, Ca, O with beneficial friction properties which are promoted by a smooth coating surface.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Turning, Material transfer, Surface topography, Coatings, Stainless steel
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-23253 (URN)10.1016/j.wear.2016.09.023 (DOI)000390733400026 ()
Available from: 2016-10-10 Created: 2016-10-10 Last updated: 2019-03-26Bibliographically approved
4. Wear mechanism of cemented carbide cutting tool in the turning of 316L stainless steel
Open this publication in new window or tab >>Wear mechanism of cemented carbide cutting tool in the turning of 316L stainless steel
2018 (English)Conference paper, Published paper (Refereed)
Keywords
Cemented carbide; stainless steel 316 L; turning; SEM; AES
National Category
Engineering and Technology
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-28052 (URN)
Conference
The 18th Nordic Symposium on Tribology – NORDTRIB 2018 18-21 June 2018, Uppsala University, Uppsala, Sweden
Available from: 2018-06-29 Created: 2018-06-29 Last updated: 2019-03-27Bibliographically approved
5. On the diffusion wear of cemented carbide in the turning of 316L austenitic stainless steel
Open this publication in new window or tab >>On the diffusion wear of cemented carbide in the turning of 316L austenitic stainless steel
2019 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 430-431, p. 202-213Article in journal (Refereed) Published
Abstract [en]

The present study focuses on the wear and wear mechanisms of three different cemented carbide grades during orthogonal turning of 316L austenitic stainless steel at different cutting speeds. The influence of WC grain size and cutting speed on the resulting crater and flank wear was evaluated by optical surface profilometry and scanning electron microscopy (SEM). The mechanisms behind the crater and flank wear were characterized on the sub-micrometer scale using high resolution SEM, energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES) and time of flight secondary ion mass spectrometry (ToF-SIMS) of the worn cutting inserts and the produced chips.

The results show that the wear rate of cemented carbide drastically increases with increasing cutting speed and that the wear is dependent on the WC grain size; i.e. the crater wear decreases with increasing WC grain size while the flank wear increases with increasing WC grain size. High resolution SEM, AES and ToF-SIMS analysis of the worn cemented carbide within the crater and flank wear regions reveal that the degradation of cemented carbide at higher cutting speeds is mainly controlled by diffusion wear of the WC-phase. This is confirmed by ToF-SIMS analysis of the back-side of stainless steel chips which reveals the presence of a 10 nm thin W-containing oxide film. The results are discussed and interpreted in the light of the conditions prevailing at the tool-chip interface.

Keywords
Cemented carbide, 316 L austenitic stainless steel, turning, diffusion wear, SEM, AES, ToF-SIMS
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-29660 (URN)10.1016/j.wear.2019.05.010 (DOI)
Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-07-24Bibliographically approved
6. Wear and wear mechanisms of cemented carbide in the turning of Ti6Al4V
Open this publication in new window or tab >>Wear and wear mechanisms of cemented carbide in the turning of Ti6Al4V
2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944Article in journal (Refereed) Submitted
Abstract [en]

Titanium and titanium alloys such as Ti-6Al-4V are generally considered as difficult-to-machine materials. This is mainly due to their high chemical reactivity, poor thermal conductivity and high strength, which is maintained at elevated temperatures. As a result, the cutting tool is exposed to rather extreme contact conditions resulting in plastic deformation and wear. In the present work, the mechanisms behind the crater and flank wear of uncoated cemented carbide inserts in the turning of Ti6Al4V are characterized using high resolution scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and high resolution Auger electron spectroscopy (AES).

The results show that for combinations of low cutting speeds and feeds crater and flank wear were found to be controlled by an attrition wear mechanism while for combinations of medium to high cutting speeds and feeds a diffusion wear mechanism was found to control the wear. For the latter combinations, high resolution SEM and AES analysis reveal the formation of an approximately 100 nm thick carbon depleted WC-layer at the cemented carbide/Ti6Al4V interface due to the diffusion of carbon into the adhered build-up layers of work material on the rake and flank surfaces.

Keywords
Turning, Cemented carbide, Ti6Al4V, Attrition wear, Diffusion wear, SEM, EDS
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-29658 (URN)
Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-07-24Bibliographically approved
7. A methodology to systematically investigate the diffusion degradation of cemented carbide during machining
Open this publication in new window or tab >>A methodology to systematically investigate the diffusion degradation of cemented carbide during machining
2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 14, article id 2271Article in journal (Refereed) Published
Abstract [en]

Using Ti6Al4V as a work material, a methodology to systematically investigate the diffusion degradation of cemented carbide during machining is proposed. The methodology includes surface characterization of as-tested worn inserts, wet etched worn inserts, metallographic cross-sectioned worn inserts as well as the back-side of the produced chips. Characterization techniques used include scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES) and time of flight secondary ion mass spectroscopy (ToF-SIMS).

The results show that the characterization of wet etched worn inserts give quick and useful information regarding the diffusion degradation of cemented carbide, in the present work the formation of a fine crystalline W layer (carbon depleted WC layer) at the tool / work material interface. The present study also illuminates the potential of AES analysis when it comes to analyzing the degradation of cemented carbide in contact with the work material during machining. The high surface sensitivity in combination with high lateral resolution makes it possible to analyze the worn cemented carbide surface on a sub-µm level. Especially AES sputter depth profiling, resulting in detailed information of variations in chemical composition across interfaces, is a powerful tool when it comes to understanding diffusion wear. Finally, the present work illustrates the importance of analyzing not only the worn tool but also the produced chips. An accurate characterization of the back-side of the chips will give important information regarding the wear mechanisms taking place at the tool rake face / chip interface. Surface analysis techniques such as AES and ToF-SIMS are well suited for this type of surface characterization.

Keywords
Turning, Ti6Al4V, Wear, Tool, Chip, SEM, AES, ToF-SIMS
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-29661 (URN)10.3390/ma12142271 (DOI)000480454300059 ()31311114 (PubMedID)2-s2.0-85070464240 (Scopus ID)
Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-08-29Bibliographically approved

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