Friction and Lubrication in Mechanical Design Episode 2 Part 10 pps

The kinetic energy of deposition is great enough to give rich plasma of ionized titanium, resulting in good adhesion at a high coating rate and low substrate temperature.. Advantages of

At the present, CVD is primarily used to coat machine tools with TIN The process starts by placing parts in a chamber and heating to 1000°C In a few hours, the parts reach a uniform temperature Gaseous chemicals are introduced into the chamber at atmospheric pressure Chemical reactions of gaseous material produce the coating material and gaseous byproducts Coating material crystallizes on the substrate surface This process takes several hours and is very sensitive to process parameters However, thick coatings can be applied by this method

12.2.2 Physical Vapor Deposition

This process relies on ion bombardment as the driving force Temperatures

are typically in the range 500-900°F for the deposition of tool coatings This

lower temperature is generally given as the major distinction between CVD and PVD processes The following are the major PVD coating processes:

1 Sputter ion plating

2

3 Arc evaporation (ion bond) Electron gun beam evaporation (ion plating)

Sputter Ion Plating

Sputter ion plating (SIP) takes place in a vacuum chamber containing argon

at a certain known pressure Parts to be coated are loaded into a standard fixture The inside surface of the SIP unit as well as all of the exposed surfaces are lined with a sheet of titanium The titanium acts as a source material The parts are held at a positive voltage (+ 900 V) with respect to titanium, resulting in a glow discharge (plasma) generated between the workload and the titanium The ionized argon bombards the titanium, sputtering titanium atoms These highly energized titanium atoms, through

a series of random collisions, migrate to the part and are deposited on the

exposed surfaces, forming a thin uniform coating Nitrogen gas is then bled

into the chamber which reacts with the deposited titanium, forming TIN

To form a fine impurity-free coating, small anodes, biased slightly higher in potential than the work load, are inserted into the chamber in close proximity to the part The effect is to produce further low-energy

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sputtering, which produces a microcrystalline structure This is due to the deposited coating itself being bombarded by high-energy argon atoms The first step in the tool coating process is to ensure that the surfaces of the components to be coated are free of oxides, rust preventatives, dust, grease, and burrs, all of which can affect adherence Cleaning of tools consists of series of mechanical/chemical treatments followed by utrasonic degreasing It is important to clean tools as carefully as possible to reduce the risk of damaging the cutting edges Cleaned tools are loaded onto simi- larly cleaned fixtures The fixture is then placed into the coating chamber, and argon gas is then flowed into the chamber The argon is purified before entering the chamber by passing over a heated titanium The tools to be coated and titanium source material are heated using external radiant hea-

ters to 300°C The pure argon sweeps away any volatile contaminants which

may be in the system or which remains on the parts

Once the chamber and the work load is at temperature, ion cleaning of the parts takes place Ion cleaning is accomplished by applying a negative

voltage (-500 V) to the parts This establishes a glow discharge in the cham-

ber from which ions are attracted to the part sputtering the surface The sputtering action provides a surface which is free of oxides or any barrier

to the coating in the chamber Ion cleaning is essential for good coating adhesion and subsequent surface performance After the ion cleaning is com- pleted, the bias is reversed, and coating is initiated, as described above

SIP is a process having excellent throwing power Because of the large

titanium source, the small mean free path of sputtered titanium, and the

operation of the system at less than 500°C (927"F), large and small tools of

different geometries may be coated in the same cycle These characteristics

of SIP set it off from other PVD processes, and clearly provide greater process flexibility

Electron Beam Gun Evaporation

This ion plating process uses a crucible of molten titanium, which is evapo- rated at low pressure by an electron beam gun to produce titanium vapor, which is attracted by an electric bias to the workpiece The process is inher- ently slow, but can be speeded up by ionization enhancement techniques The main drawback is the constraint that the workload must be suspended above the melting crucible, using water-cooled jigging, and uniformity is difficult to achieve

Arc Evaporation

With the arc evaporation (ion bond) method ARE, blocks of solid titanium

are arranged around the chamber walls and an arc is struck and maintained

Surface Coating 45 7

between the titainum and the chamber Titanium is evaporated by extreme local heat from the arc into the nitrogen atmosphere and attracted to the workpiece The main advantage is that evaporation occurs from the solid rather than the liquid phase Arc sources therefore may be placed at any angle around the workpiece, which is simply placed on a turntable in the base of the chamber Thus uniformity is achieved without complex jigging The kinetic energy of deposition is great enough to give rich plasma of ionized titanium, resulting in good adhesion at a high coating rate and low substrate temperature Parameters such as coating thickness, coating composition and substrate temperature are easily controlled

12.2.3 Comparison between the CVD and PVD Processes

The principal difference between CVD and PVD processes is temperature This is of major significance in the coating of high-speed steels as the 1750-

1950°F of CVD exceeds the tempering temperature of HSS steel, therefore,

the parts must be restored to the proper condition by vacuum heat treat- ment following the coating process With properly executed heat treatment, this generally causes no problems However, in some cases involving extre- mely fine tolerances, post-coating heat treatment does produce unacceptable distortion

The high temperature of the CVD process makes it somewhat less demanding than PVD in terms of cleanliness of the workpiece going into the reactor: some types of dirt simply burn off Additionally, high tempera- tures tend to ensure a tightly adhering coating, and PVD temperatures are often pushed to the HSS tempering range to enhance coating adhesion

CVD coatings tend to be somewhat thicker (typically 0.0003in.) than those deposited by different PVD processes (often less than 0.0001 in thick) This may be advantageous in some cases, disadvantageous in others Another characteristic of the CVD coating is that they yield a matte surface somewhat rougher than the substrate to which they are applied If the application requires it, this can be polished to a high luster, but this is an extra step PVD coating on the other hand faithfully reflect the underlying surface

Because the CVD reactions take place within the gaseous cloud, every- thing within that cloud will be coated This permits workpieces to be closely packed within a CVD reactor with complete coating of all surfaces except those points on which the parts rest Even deep cavities and inside diameters will become coated in a CVD reactor

Except for SIP, all PVD processes have limited throwing power PVD reactors are therefore less densely packed, and the jigging and fixtures are more complicated

458 Chapter I2

A cost comparison between CVD and PVD is complex The initial

investment in the equipment is as much as three to four times as great for PVD as for CVD The PVD process cycle time can be one tenth that of CVD Mixed components can be coated in one CVD cycle, whereas PVD is much more constrained

The main advantage of the PVD process is that most metallic and ceramic coatings can be deposited on almost any substrate The process is very flexible Several process parameters can be controlled directly and the process is insensitive to slight variation of process parameters The process is fast and relatively inexpensive because vaporized coating material is carried directly to the substrate where particles condense to form a film

12.3 TYPES OF COATINGS

Surface coatings can be divided into two subgroups, hard and soft coatings Hard coatings are recommended for heavy load or high-speed applications Beneficial characteristics are low wear and long operating periods without deterioration of performance Hard coatings include iron alloys, ceramics like carbides and nitrides, and nonferrous alloys

Soft coatings are recommended for low-load, low-speed applications Advantages of soft coatings are low friction, low wear, and a wide range of operating temperatures Soft metals have received much attention because

of their low-load, friction-reducing properties Many soft coatings are actu- ally solid lubricants (like graphite) that require a resin binder to adhere to the surface These coatings are typically applied to protect parts during a running in period

12.3.1 Soft Coatings

Soft coatings can be grouped into four main categories: layered lattice com- pounds such as graphite, graphite fluorides, and MoS,; nonlayered lattice compounds such as PbO-Si02, CaF,, BaF2, and CaF2-BaF2 eutectics; polymers; and soft metallic coatings [ 11

Layered Lattice Coatings

Most layered lattice coatings are hexagonal compounds with slip planes that are oriented parallel with the surface These compounds are like plates stacked up on top of each other The plates slip easily when subjected to a shear force However, they resist movement normal to the surface Burnishing is an important step that aligns the plates parallel with the surface

Surface Coating 459

The most common layered lattice compounds are graphite, graphite fluorides, and MoS2 Graphite and graphite fluoride compounds tend to perform better at room temperature in humid environments Current under- standing is that adsorbed moisture helps the plates slip Higher temperatures drive off moisture and explain a rapid increase in the friction coefficient Above 430°C, the friction coefficient drops It is believed that graphites interact with metal oxides that form on the mating surface to reduce friction Graphite fluorides generally perform better than pure graphite but have a life about ten times longer at room temperature They are not sensitive to humidity and operate well in a vacuum However, unlike graphite, wear life decreases proportionally with temperature rise The compound decomposes around 350°C

Nonlayered Lattice Coatings

These coatings are based on inorganic salts The main characteristic is a phase change caused by frictional heating The coating is solid at the bulk temperature but becomes a high viscosity melt at the friction interface Advantages are chemical inertness and effectiveness at high temperatures Some fluoride salts remain effective at temperatures approaching 900°C Disadvantages are high friction at low temperatures and manufacturing difficulties These coatings are very difficult to apply to substrates

Polymer Coatings

Polymer coatings are applied to metal and nonmetal surfaces by several different techniques Traditionally, polymers are used to repel water and resist corrosion They can resist erosive, abrasive wear caused by impacting particles because the coating is elastic The coating deforms to absorb par- ticle impact, then returns to its origial shape Friction is typically very low, especially when polymers are applied to hard substrates

Polymers are used by industry for bearings, automotive components, pumps, and seals The coatings are inexpensive and easy to apply Wide use

by industry has helped build a large base of empirical knowledge

Soft Metal Coatings

Soft metals are compatible with liquid lubricants, effective at low tempera- tures and at elevated temperatures (silver and gold are effective near their melting points), can operate in a vast range of normal pressures from vacuum

to high pressure, and perform well at high speeds Soft metals can be applied

by several different processes However, high material costs limit widespread use of soft metals Bhushan [ 11 compiles results of several studies performed

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with ion-plated soft metal coatings Tests were performed with a pin-and-

disk apparatus The disk was coated; the pin was not A common character-

istic is dependence of friction coefficient on coating thickness where friction reaches a minimum at a critical coating thickness In the ultrathin region, surface asperities of the mating surface break through the coating and inter- act with the substrate Thus in the limit, the friction coefficient reaches that of the substrate material In the thin region, the real and apparent areas of contact are equal, leading to an increase in friction with increasing coating thickness and reaches an asymptote for thickness above 10 pm

Studies on the effect of sliding velocity on the friction coefficient and wear life of silver, indium and lead suggest that velocity has little effect on the coefficient of friction A slight decrease in friction at higher speeds may occur due to thermal softening of the coated material On the other hand, sliding velocity has a large effect on wear life Sherbiney [2] reported that wear life is inversely proportional to speed More recent studies reported by Bhushan indicate that soft metal coatings alloyed with copper or platinum tend to improve wear life and reduce friction [l]

12.3.2 Hard Coatings

Hard coatings are recommended for heavy-load, high-speed applications These coatings exhibit low wear, can be used for long periods without deterioration in performance, and protect against wear and corrosion in extreme conditions The main types of hard coatings are ferrous alloys, nonferrous alloys, and ceramics Table 12.1 lists common hard coatings and general properties

Iron alloys are generally hard and brittle Steel alloy coatings are more ductile and better able to resist mechanical shock Nonferrous alloys are primarily used for corrosion resistance at high temperatures Ceramic coat- ings are hard, brittle, chemically inert, and against corrosion Titanium- based ceramic coatings are revolutionizing the machine tool industry

Iron-Based Alloys

Iron based alloys are usually applied by weld deposition or thermal spray- ing Alloying with cobalt improves oxidation resistance and hardness at elevated temperatures High chrome and martensitic irons are hard, not

as tough as steel coatings Martensitic, pearlitic, and austenitic steels are recommended for heavy wear and conditions where mechanical or thermal shock are expected The irons resist abrasion better, but are not recom- mended for applications involving mechanical and thermal shock

Tungsten carbides Maximum abrasion resistance, worn

surfaces become rough High-chromium irons Excellent erosion resistance, oxidation

resistance Martensitic irons Excellent abrasion resistance, high

compressive strength

Co bal t-based alloys Oxidation resistance, corrosion resistance,

hot strength and creep resistance, composition control, several options for coating deposition, good galling resistance

Corrosion resistance, may have oxidation and creep resistance, compositional control, several options for coating processes, relatively inexpensive, poor galling resistance

impact resistance, good compressive strength

resistance maximum toughness with fair abrasion resistance, good metal-to-metal wear resistance under impact

abrasive wear, low friction coefficient, good corrosion resistance

High hardness, good abrasion resistance, brittle, low friction coefficient, low wear, corrosion resistant, can be applied to some plastics, weakly ferromagnetic

Hard Coating Reference Chart

Good combinations of abrasion and Inexpensive, fair abrasion and impact Work hardening, corrosion resistance,

Good thermal conductivity, resists

Chrome- Based Coatings

Chrome alloys are usually applied by electrochemical deposition and PVD CVD less commonly used Electrochemically deposited chrome has a hard- ness around 1000 HV that is stable up to 400°C Th electrochemical process

is very slow, thus more costly

462 Chapter 12

The preferred method of applying chrome coatings is PVD Hardness of

pure chromium coatings can reach 600 HV Doping with carbon or nitrogen

produces hardness of 2400 HV and 3000 HV respectively

Nickel Coatings

Nickel applied by electrochemical deposition is one of the oldest known coating methods This coating is primarily used for corrosion protection and decorative artifacts

Electrolysis-deposited nickel coatings are better for wear and abrasion resistance With the addition of phosphorus or boron, hardness can reach

700 HV The tradeoff is slightly less corrosion resistance Heat treatment

after the deposition process promotes the formation of nickel borides or nickel phosphides, which increases hardness Adding particles of solid lubri- cant helps reduce the coefficient of friction

Cobalt-Based Alloy Coatings

Cobalt-based alloys are hard and ductile Uses include high-temperature wear, mild abrasion resistance, and corrosion resistance With the addition

of ceramic carbides such as tungsten carbide, chromium carbide, and coblat carbide, the coating can be used to temperatures of 800°C Common deposi- tion techniques are welding and plasma spray

Nickel-Based Alloy Coatings

Nickel-based alloys were developed as a substitute for cobalt-based alloys Nickel is much cheaper than cobalt coatings, yet has similar characteristics

Ceramic Coatings

Common techniques for depositing ceramic coatings are thermal spray,

PVD, and CVD Ceramic coatings can be applied to metals, ceramics,

and cermets The most common ceramic coatings are oxides, carbides, and nitrides (Table 12.2) Another form of ceramics, hard carbon coatings (graphite based and diamond based), began receiving much attention in the last ten years At present, titanium nitride (TIN) is the most studied and the most used ceramic coating It gained acceptance in the machine tool indus- try because of its high hardness, low friction, chemical inertness in the presence of acids, and extremely long wear life

Titanium carbide (Tic), Aluminum oxide (A1203) and Hafnium nitride

(HfN) are also in use The use of multiple layer coatings such as TiN over

Tic, and A1203 over TIC is also been made Triple coatings of TiC/A1203/ TIN have been proved beneficial, exploiting the characteristics of all three

Surface Coating 463

Table 12.2 Common Ceramic Coatings

CVD

CVD

temperatures, low friction coefficient Soft coating used for corrosion resistance Deposited on other alumina coatings listed to improve corrosion resistance

Excellent wear resistance at ambient and elevated temperatures, thick coatings tend to spall, thin coatings show good substrate adhesion, low friction coefficient at high temperatures

HSS tools, very low friction and wear in dry and lubricated conditions, hardness and adhesion is

a function of substrate temperature during deposition

Hard, brittle, excellent wear resistance, excellent adhesion with substrate, very low friction, couple with TiN and Sic to improve friction and wear

extremely hard

PVD CVD Plasma spray, radio frequency sputtering Chromia

Maintains hardness at elevated temperatures, Low deposition temperature, hard, brittle, sensitive to thermal and heavy load cycling

464 Chapter I2

Table 12.2 Continued

~

Silicon

CVD CVD, ion plating

PECVD, sputting PVD

Pure chromium has high friction Nichrome coatings moderate ductility, good adhesion with substrate, high dependency of friction on sliding speed, hardness approaches 500 HV

Limited studies performed High hardness (up to 6000HV), good oxidation resistance at high temperatures, chemically inert

in contact with acids, thermal stability increases with carbon content, requires diffusion barrier between coating and substrate if used on steels, high friction

Low friction and wear, chemically inert, excellent adhesion with substrate, high-temperature deposition process removes temper from high- strength tool steels

Smae as CVD but can be deposited at much lower temperatures

Low friction and wear with lubrication, high friction and wear when dry, friction and wear not affected by humidity, chemically inert, excellent adhesion to substrate

High hardness of nitrides at temperatures above

8OO0C, hardness decreases with temperature,

low thermal conductivity (thermal barrier coating for HSS tools)

Good oxidation resistance, good erosion resistance, low thermal expansion, poor adhesion with substrate at elevated temperatures

Not studied as extensively oxides, carbides, and nitrides High hardness, high melting point, corrosion resistant, abrasion resistant

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coatings Generally, these coatings applied to cutting tools can extend useful tool life of cutting tools and wear parts by as much as 300% or more But the search for even more productive, cost-efficient tooling continues New coatings, such as titanium diboride, silicon carbide, and silicon nitride are being explored and more effective coating processes are being developed One disadvantage of ceramic coatings is the deposition process Depositing ceramic coatings on substrates is difficult because several vari- ables affect quality and some of them have shown that ceramics are extre- mely sensitive to the deposition process and substrate temperature Researchers are investigating solutions to these problems Meanwhile, coat- ing manufacturers rely on trial and error to find process variables that work

12.4 DIAMOND SURFACE COATINGS

Diamond is the ultimate substance for wear resistance Desirable properties are low friction, extremely high thermal conductivity, low electrical conduc- tance, high wear resistance, and high abrasion resistance One disadvantage

is that diamond coatings do not adhere well to substrates, and the finished coated surface can be extremely rough Technical problems associated with producing these coatings stem from the extreme temperatures and pressures

necessary to produce tetrahedral carbon bonds In the early 1970s, research-

ers discovered deposition techniques that produced diamond-like coatings at

relatively low pressure and temperature (around SOOOC) Recently, a 400°C

process was discovered, sacrificing deposition rate

These techniques produced diamond-like films from hydrocarbon gases

It is easy to form graphite-like, three-bond structures from hydrocarbons, but difficult to go the next step from layered hexagonal bonds to the tetra- hedral bonds found in diamonds Thus, the final structure of the coating is a hybrid of graphite- and dimaond-like bonds The resulting physical proper- ties are between those of graphite and diamond films For example, dia- mond-like coatings decompose around l OOO'C, between the decomposition temperature of graphite and diamond This holds true for other physical properties such as thermal conductivity, electrical conductance, and friction coefficient Current research is focusing on coating techniques and substrate preparations that can improve the process of diamond and diamond-like coatings

12.4.1 Properties of Diamond

Diamond is an exceptional material Most of its important properties can be labeled as extreme It has the highest hardness, the highest thermal conduc-