| Abstract | Tvrdi metali, komercijalnog naziva Widia, u industriji obrade odvajanjem čestica
najistraživaniji su i najrazvijeniji predstavnici materijala dobivenih metalurgijom praha. Razvoj
ovih materijala u današnje vrijeme zasniva se na primjeni ultrafinih i nano čestica praha za
sinteriranje kojima se postiže znatno poboljšanje svojstava proizvoda, omogućuje se primjena
pri većim brzinama rezanja, manje tolerancije i duži vijek trajanja alata. Najvažniji način
povećanja tribološke otpornosti alata svakako je prevlačenje istih tankim tvrdim prevlakama.
Prevučeni tvrdi metali trenutno predstavljaju 80-90% proizvodnje reznih alata zahvaljujući
jedinstvenoj kombinaciji otpornosti na trošenje i žilavosti kao i mogućnosti oblikovanja
složenih oblika. Poznata je činjenica da je površinskim postupcima (modificiranjem i/ili
prevlačenjem) moguće formirati sustav površinski sloj/osnovni materijal sa svojstvima koja
mogu zadovoljiti suvremene zahtjeve uz prihvatljive proizvodne troškove.
Ipak, komercijalno dostupni tvrdometalni alati još nisu zakoračili u nano područje te im je
veličina zrna karbidne faze nakon sinteriranja veća od 200 nm. Razlog tome leži u trenutnoj
visokoj cijeni proizvodnje nano zrnatog tvrdometalnog alata, kompleksnosti postupka
sinteriranja te strogo kontroliranim laboratorijskim uvjetima i visokoj čistoći okoliša. Sukladno
tome i postupci prevlačenja koji se primjenjuju na takvim alatima relativno su starijeg datuma
jer su se u praksi pokazali dostatnim za stvaranje kompaktnog sloja osnovni materijal -
prevlaka. Kako su nano materijali veoma reaktivni, kako u fazi sinteriranja, tako i u fazi
prevlačenja, prevlačenjem istih postupcima kemijskog isparavanja iz parne faze (eng. CVD) pri
oko 1000 °C na površini alata formiraju se mikrostrukturni defekti koji narušavaju jedno od
najvažnijih svojstava istih, a to je žilavost. Sniženje temperature prevlačenja omogućuje
primjena jedne od najsuvremenijih tehnologija inženjerstva površina tj. tehnologija plazmom
potpomognutog prevlačenja iz parne faze, (eng. PACVD). PACVD postupkom, danas još
uvijek nedovoljno istraženim kad je riječ o tvrdim metalima, upotrebom plazme snižava se
temperatura prevlačenja na oko 500 °C.
U okviru ovog doktorskog rada uspješno je sinteriran i proizveden tvrdometalni alat s
masenim udjelima kobalta 5, 10 i 15 % na kojem su plazmom potpomognutim prevlačenjem
iz parne faze s ciljem poboljšanja svojstava ovih alata razvijeni novi, inovativni površinski
slojevi koji čine sustav osnovni materijal - prevlaka do sada nepoznat i neistražen u praksi.
U teorijskom dijelu rada detaljno je opisan postupak dobivanja sinteriranog tvrdometalnog
proizvoda, opisani su tehnički zahtjevi koji se postavljaju na rezne alate u pogledu njihovih
svojstava i objašnjeni su mehanizmi dotrajavanja tvrdometalnih alata. Detaljno je opisan
PACVD postupak iz perspektive nanošenja prevlaka na tvrdi metal i sami mehanizmi
nastajanja istih.
Eksperimentalni dio podijeljen je na dva dijela od kojih se prvi odnosi na provođenje i
validaciju uspješnosti sinteriranja tvrdog metala nano veličine zrna, a drugi uključuje nanošenje
prevlaka na proizvedeni materijal.
U prvoj cjelini eksperimentalnog dijela detaljno su opisane karakteristike polaznih
mješavina prahova, kao i postupci koji su prednjačili sinteriranju naprednim postupkom sinter
HIP-a. Formiranje zrna nano veličine, bez prisutnosti neželjenih mikrostrukturnih defekata,
potvrđeno je XRD ispitivanjem, mikrostrukturnom analizom i određivanjem veličine
karbidnog zrna na skenirajućem elektronskom mikroskopu s emisijom polja (FESEM), ali i
indirektno nerazornim metodama karakterizacije kao što je mjerenje magnetskih svojstava
tvrdih metala. Od mehaničkih svojstava ispitana su najvažnija svojstva tvrdih metala, a to su
tvrdoća i lomna žilavost, te su dobiveni rezultati uspoređeni s literaturnim vrijednostima za
tvrdometalne materijale identične veličine zrna karbidne faze.
Drugi dio uključuje postupak PACVD prevlačenja s naglaskom na razvoj novih inovativnih
sustava prevlaka - osnovni materijal. Određivanjem kristalne strukture XRD metodom
potvrđeno je da PACVD postupak ne uzrokuje stvaranje mikrostrukturnih defekata na površini
osnovnog materijala. Karakterizacija dobivenih površinskih slojeva prevlake provedena je
mikrostrukturnom analizom pomoću svjetlosne i elektronske mikroskopije, određivanjem
hrapavosti površine, ispitivanjem prionjivosti prevlaka metodom brazdanja i metodom
utiskivanja Rockwell C indentora, mjerenjem debljine prevlaka metodom kalotest i
ispitivanjem mikrotvrdoće prevučenih slojeva.
U cilju kvantificiranja utjecaja novorazvijenih površinskih slojeva na svojstva i trajnost
tvrdometalnih alata provedeno je ispitivanje erozijskog trošenja (erozija česticama),
određivanje faktora trenja i kliznog trošenja (kuglica na ploči), ali i eksploatacijsko ispitivanje
trajnosti alata pokusom tokarenja na obradnom centru.
Na osnovi sveobuhvatne analize dobivenih rezultata zaključno je potvrđeno da se PACVD
prevlakama može ostvariti značajno poboljšanje triboloških i mehaničkih svojstava reznih alata
od nanostrukturiranih tvrdih metala. |
| Abstract (english) | Hardmetals, commercially called Widia, are the most researched and developed
representatives of powder metallurgy materials in the metal cutting industry. The development
of these materials nowadays is based on the application of ultrafine and nano sized powders for
sintering which significantly improve the properties of the product, allow application at higher
cutting speeds, lower tolerances and longer tool life. The most important way to increase the
tribological properties of tools is certainly to coat them with thin hard coatings. Coated
hardmetals currently account for 80-90% of cutting tool production thanks to their unique
combination of wear resistance and toughness as well as the ability to form complex shapes.
By surface processes (modification and / or coating) it is possible to form a surface layer / base
material system with properties that can meet modern expectations with acceptable production
costs.
However, commercially available cemented carbide tools have not yet stepped into the
nano range and their grain size of the carbide phase after sintering is higher than 200 nm. The
reason for this lies in the current cost of production of cemented carbide tools below the nano
grain size, the complexity of the sintering process, and the high purity of the environment in
which such tools are produced in laboratory conditions. Accordingly, the coating procedures
used on such tools are relatively older because in practice they have proven to be sufficient to
create a compact layer of base material - coating. As nano materials are highly reactive, both in
the sintering and coating phase, when using chemical vapour deposition process (CVD) at
around 1000 °C on the surface of the tool, microstructural defects are formed that disrupt one
of their most important properties, which is toughness. The reduction of the coating temperature
is enabled by applying of one of the most modern surface engineering technologies, ie. plasmaassisted
chemical vapour deposition (PACVD) process. The PACVD process, which is
nowadays still insufficiently studied on hardmetals, uses plasma to lower the coating
temperature to about 500 °C.
In the framework of this doctoral thesis, a cemented carbide tool with 5 wt. % Co, 10 wt.
% Co and 15 wt. % Co was successfully sintered, and by the use of plasma-assisted chemical
vapour deposition, with the aim of improving properties of these tools, new and innovative
systems base material/coating were produced, which were until now unknown and unexplored
in practice.
The theoretical part of the paper describes in detail the process of obtaining a sintered
cemented carbide product, describes the technical requirements that are placed on cutting tools
in terms of their properties and explains the mechanisms of wear of cemented carbide tools.
The PACVD process is described in detail from the perspective of coating hard metals and the
mechanisms of forming layers have been explained.
The experimental part is divided into two parts, the first of which refers to the
implementation and validation of the success of sintering of nano-sized hardmetal, and the
second involves the application of coatings on the produced material.
In the first section of the experimental part, the characteristics of the initial powder
mixtures are described in detail, as well as the procedures that preceded the sintering by the
Sinter HIP process. Nano-sized grain formation, without the presence of unwanted
microstructural defects, was confirmed by XRD testing, microstructural analysis and carbid
grain size determination on a scanning electron microscope with field emission (FESEM), but
also by indirect non-destructive characterization methods such as measuring magnetic
properties. Of the mechanical properties, the most important properties of hard metals were
examined, namely hardness and fracture toughness, and the obtained results were compared
with the literature values for hardmetal materials of identical grain size of the carbide phase.
The second part includes the PACVD coating process with an emphasis on the
development of new innovative coating systems, coating/base material. By determining the
crystal structure by the XRD method, it was confirmed that the PACVD process does not cause
the formation of microstructural defects on the surface of the base material. Characterization of
the obtained surface layers was performed by microstructural analysis using light and electron
microscopy, surface roughness determination, coating adhesion testing by scratch test method
and Rockwell C indentation method, coating thickness measurement by calotest method and
microhardness test of coatings.
In order to quantify the impact of newly developed surface layers on the properties and
durability of cemented carbide tools, erosion wear testing (particle erosion), determination of
friction and sliding wear factors (pin on disc), but also testing of tool durability by single point
turning test was performed.
Based on a comprehensive analysis of the obtained results, it was concluded that PACVD
coating process can achieve significant improvements in the tribological and mechanical
properties of cemented carbide cutting tools. |
