Manufacturing Engineering 2 substrates and coatings 5.1

what is High-speed steel (HSS)?

-tool steels with alloy content up to 35% to improve wear resistance and hot hardness.

How is HSS made

1.metallugically smeltered

2.hardened and quenched to dissolve most carbides

3. annealed to reduce martensitic phase, increasing toughness

what is HSS used for

often considered the cheap option, but prioritised for tools that need very high toughness or need to exhibit very small and fragile cutting teeth

Cemented tungsten carbide good

exhibit higher hot hardness and wear resistance, but less tough

what is cemented tungsten carbide comprised of and how made

tungsten carbide in a cobalt matrix, produced by powder metallurgy, a process called sintering, that bakes the constituents together

3 basic grades of tungsten carbides

K, P, M

What is K grade and what material is it used for?

WC, used for non-ferrous metals, ferrous metals with short chips, cast iron

What is P grade and what is it used for?

WC+ TiC, used for ferrous metals with long chips: low-carbon, stainless and other steels

What is M grade and what is it used for?

WC + TiC + TaC, intermediate category for ductile irons, hard steels and high-temperature alloys

How does increasing the concentration of carbides affect the tool

increases wear resistance as it reduces diffusion of carbon, decreasing rate of carbon depletion in the tool

also becomes more brittle due to reduction in cobalt

What does increasing TaC do

promoted bending strength as it promotes the creation of finer grains, but also reduces substance hardness due to being softer than WC

What does increasing cobalt concentration do and why

lowers hardness and wear resistance, but increases bending strength. This is because cobalt is less hard than WC, but is the bonding phase.

how else can we change the composition of cemented tungsten-carbide tools

adjust size of carbide grains into fine-grain and ultra-fine-grain, whee decreasing grain size results in increases in wear resistance and hardness

How are cemented tungsten-carbide tools made

they are sintered

steps to making tungsten-carbide tools

weighing of ingredients, milling, blending, compaction, soft machining, sintering

what does sintering do

it is heating the compacted powder below melting point, but high enough to cause bonding between particles

what are ceramics

they are composed primarily of fine-grained, high-purity Al2O3, pressed and sintered with no binder

what are the 2 types of ceramics

-white, or cold-pressed,which consist of Al2O3 cold-pressed into inserts and sintered at high temps

-Black, or hot-pressed, which consists of 70% Al2O3 and 30% TiC

what properties do they exhibit

high-hot hardness chemical stability and wear resistance, but low resistance to mechanical and thermal shock

Tool coating purpose

prolong life of tool, or reduce adhesion tendency of workpiece material to the tool surface, reducing cutting forces and improve chip flow for drilling

types of tool coating

soft and hard coatings

purpose of hard coatings

shield tool substrate against wear

purpose of soft coatings

to reduce friction between cutting tool and surface

2 ways of putting coatings on tools

chemical vapour deposition and physical vapour deposition

how is CVD carried out and what does it achieve

▪ CVD is an atmosphere-controlled process conducted at elevated

temperatures (~1,000°C) in a CVD reactor.

▪ During this process, thin-film coatings are formed as the result of

reactions between various gaseous phases and the heated

surface of substrates within the CVD reactor.

▪ As different gases are transported through the reactor, distinct

coating layers are formed on the tooling substrate.

▪ As for example:

▪ TiN: 2TiCl4 + N2 + 4H2 → 2TiN + 8HCl.

▪ TiC: TiCl4 + CH4 + H2 → TiC + 4 HCl + H2.

▪ The final product is a coating that exhibits a strong chemical and

metallurgical bond to the substrate.

How is PVD carried out

▪ Coating process is carried out in a vacuum chamber.

▪ First, the substrate is bombarded by argon ions to create a

contamination-free tool surface.

▪ Then, high purity, solid coating material, the so-called target, is

either evaporated by an arc ('arc evaporation') or by bombardment

with argon ions ('sputtering'). Target material can be, e.g.,

titanium, chromium, aluminium.

▪ The atoms of the target material then travel, guided by an electric

field, to the substrate. At the same time, a reactive gas (nitrogen or

a gas containing carbon) is added; it forms a compound with the

atoms, which settles on the tools as a thin coating.

▪ Due to the movement of the target atoms, PVD is a line-of-sight

process that requires moving fixtures to ensure uniform coating

thickness.

Good Bad CVD

+Good layer adhesion

+Consistent layer distribution, no shadowing effect

-High process temperatures might cause thermal damage to substrate

-few suitable materials for coatings as material is fed in gaseous form

Good Bad PVD

+Wide range of suitable materials

+low coating temperature unlikely to cause damage to substrate

+multi-layer coatings are possible

-coating of internal surfaces is tough

-surface requirements of substrate is much higher

-in the case of arc evaporation, droplets can occur