PVD TECHNOLOGY

BEGINNING OF PVD COATINGS

The development of  PVD (Physical Vapour Deposition) coatings began in the early  70’s with an aim to depositing high-hardness layers improving wear-resistance. These layers were initially made out of ceramic compounds, the latter bearing one single metal. The most popular one was TiN (Titanium Nitride) and so, its characteristic golden color was present in every type of cutting tools, hardmetal plates, drill bits, mills, thread inserts, saw blades and so on .

The high possibilities of depositing compounds close to diamond hardness generated great expectations. Thus, new compounds were developed with different PVD techniques innovating the way of the evaporation of the reactive metals.

The hard coatings are mostly  based on nitrides (metal with nitrogen), carbonitrides (metal combined with nitrogen and carbon), carbides (metal and carbon) and oxides (metal and oxygen). The progressive evolution of the coatings in the global-market is as follows: gradual layers, multilayers and layers at the nanometric scale.

In the mid 90’s the titanium and aluminium nitride-based coatings made their first appearance. Its high hardness and high oxidation resistance proved quite a notable advance in the PVD industrial applications.

Evaporating two or more metals at the same time  in specific percentages was a filter for the technologies that could not be easily obtained e.g the  E-Beam  (evaporation through beams of electrons). Electric arc evaporation  and cathodic pulverization (magnetron sputtering) are the technologies that currently perform better for a new generation of coatings.

 

TECHNIQUES FOR OBTAINING PVD COATINGS

Current PVD techniques are listed as follows: E-beam, Cathodic Arc and cathodic pulverization (Sputtering).

The metal evaporation and ionization during the coating stage is carried out via physical means.  Hence the term physical vapor deposition.


When the metal is ionized, the deposition is produced by reacting the already ionized metal with N or C, or both of them (also ionized). Once in plasma state, the specimens are condensed onto the sample which is polarized for generating ion-attraction and  high density coating.

In all techniques the usual stages are as follows:

  • Loading of reactor
  • High vacuum (10-5 mbar minimum)
  • Heating
  • Ion etching
  • Coating
  • Cooling and  unloading

 

If the evaporation is produced by the effect of an electric arc that is displaced through the top of the metal (cathode), then we are dealing with arc evaporation. If  the evaporation is produced by ion-beam bombardment of argon over the metal or ceramic target the process is then called Sputtering (in this particular case the target or cathode does not have to be metallic). The ionization can be optimized by applying magnetic fields (magnetron) on the target.

As soon as the reactive ions are generated, the very-low-pressured gases are introduced into the reaction chamber so as to form the compounds. A difference in potential (continuous or pulse) is applied between the samples and the reactor walls to attract the ions to the pieces.

These techniques are occasionally confused with treatments in chemical, electrolytic or galvanic baths carried out in open installations. The PVD installations are hermetic, coat at low pressures  (10-2-10-3 mbar) and the reactions take place thanks to ionizing the reactants (plasma state).

 

PVD MAGNETRON SPUTTERING

The difference between the cathodic-pulverization evaporation (PVD MS, magnetron sputtering) and the other PVD technologies lies essentially in the manner the metal or ceramic compound is evaporated.

PVD MS is produced by means of ion-bombarding an inert gas, e.g. argon, over the target. Fig.1.

This bombardment is amplified and densified owing to the usage of strategically designed magnetic fields. In this way, the metallic atoms can be sublimated and ionized without passing through the melting state. In the advantages section, the importance of avoiding the already-mentioned state is depicted.

When the metal or ionic compound is obtained, the deposition process is similar to all the PVD techniques thereby the coating thickness will depend upon the movement characteristics of the reactor’s samples and the coating time.

 

ADVANTAGES OF PVD MS TECHNOLOGY

From the scientific and academic point of view, the “sputtering” technology has always been the most studied in the department of obtaining highly homogeneous layers that showed constant composition and morphology. Likewise, it has been broadly studied on the account of granting the possibility of evaporating exotic, ceramic, non-conductors materials among others.

Nevertheless, the layer’s growth was initially pretty slow thus making it almost not industrially applicable. The advances in unbalanced magnetrons, magnetic fields’  innovations and pulsed supply of the ionic bombardment over the targets and substrates (variant called HIPIMS, High Power Impulse Magnetron Sputtering) has allowed the obtaining of thickness in slightly higher time than that of the arc evaporation. Furthermore, this process is the most extended when speaking in industrial terms.

 

Tecnologia-PVDMS_fig1

Fig 1. Basic mechanism of the cathodic-pulverization evaporation “sputtering”

 

The sublimation of ions without passing through the fusion state allows to obtain homogeneous layers that show no discontinuities due to the formation of metal-melted microdroplets characteristics of other evaporation systems e.g. arc evaporation (fig 2 y fig 3).

Flubetech_arco Flubetech_MS

 

 Fig. 2 and Fig.3.  Common Arc electric PVD surface (left) and PVD magnetron sputtering surface.

 

 

 

The surface exact replica that PVD MS obtains, allows to coat moulds that present a spectacular polishing, even optical gloss.

Another advantage the system of evaporation through ionized gas presents is the possibility of evaporating ceramic materials. In contrast to other PVD processes, there is no need to use conductors materials as targets. Some of the ceramic compounds that can be easily obtained by means of PVD MS are titanium diboride, aluminium oxides, graphite (DLC) amongst others.

Graphite, which  is found amidst the subliming materials, can be combined with an hidrocarbonous atmosphere in order to obtain DLC layers of very low friction coefficient, high hardness (up to 6000 HV) and an excellent resistance towards corrosion .

 

PROPERTIES OF PVD COATINGS

The coating’s features is based on the tribological enhancement of the surface, friction coefficient and corrosion resistance amongst others. The summary of the aforesaid values are listed in table 1.


 

Hardness

On one hand, harndness can be measured by nanoidentification and on the other hand, the hardness related to the common compounds is widely known. Taking into account that diamond is the hardest natural element whose assigned hardness value is 10000 HV in Vicker’s scale, PVD compounds range between 2500 HV and 4000 HV.

 

Friction

While hardness relates with the resistance to abrasive wear, tribology and friction refers to the gliding capacity and avoiding microsoldering between two in-contact materials. The friction coefficient helps us elucidate the relationship between the efforts that make layers slide against each other. Hard layers impede the friction against metals and thus, the friction coefficient is greatly lowered.  Additionally, layers are capable of reproducing a polished surface if we count that there is a lubricant coating (Carbon, Sulphurs and some metals), then the friction diminishes down to a point of avoiding adhesive wear (hot and cold seize). The values illustrated in the following table are those obtained at temperature and humid conditions of 20-23ºC and 85%, respectively.

Coating

Hardness HV/GPa

Friction Coefficient

Colour

Corrosion Resistance

TIN

2200/ 22

0.3

Golden

Good

AlTIN

3200/ 30

0.5

Dark purple

Good

AlTICRN

3700/ 38

0.5

Dark grey

Good

DLC

2700/ 28

0.1 (0.01 DLC- DLC)

Anthracite Black

Excellent

TiB2

2200/22

0.5

Metallic

Good

ZrN

3700/38

0.5

Weak Golden

Very Good

CrN

2400/24

0.3

Metallic

Very Good

 Table 1. Properties of the distinct PVD MS compounds

 

Corrosion resistance

PVD MS are not, per se, andticorrosive coatings. Nevertheless, every defect-free layer that seals the surface improves the resistance to corrosion. Moreover, there are both acid and basic, inert compounds which generate stable oxides that help the sealing. Carbon-based compounds like DLC, chromium-based compounds like CrN and zirconium-based compounds like ZrN present improvements with respect to the more conventional yet functional coatings.

 

Surface finish, Roughness

The ionization without passing through the melting of the metal, which is typical from PVD MS, allows to copy the surface with absolute fidelity. Not only it does the absence of microdroplets help to obtain mirror surface, but also impedes the cracking propagation initiated in them under compression or shear stress.

Friction coefficients are intimately linked with the compounds’  tribologic capacity but the initial finish of the surface to coat and the final coating’s finish are fundamental (Fig. 4 and Fig. 5).  Clearly, makes the PVD MS technology, the one with the best imminent future.

PVD MS technology is leader among PVD techniques.

Tecnologia-PVD-fig4-150x150 DLC

T

Fig.4 and Fig.5 Surface roughness shown by optical microscopy (left) and SEM (right)

 

 

The development of HIPIMS technology, the combination with pulsed polarized sources, the quality of the coating’s finish, the possibility of evaporating a bigger number of different compounds and the growth in the form of dense supernitrides makes the PVD MS technology, the technology with a better imminent future and a long-term career in so different sectors viz. decorative, mechanization, polymeric injection and hot and cold metal stamping.

 

APPLICATIONS OF PVD COATINGS

Currently, PVD coating are successfully being applied in cutting tools, extrusion, lamination and cutting dies, polymer-injection moulds of polymers, light alloy-injection moulds (aluminium, Zamack, magnesium and so on), surgical tools, prosthesis, implants and wear-submitted components.

For further information about PVD applications, please visit Moulds‘, Dies’, Cutting Tools’ Biomedics’ Applications .