Lesson 9.4ShapeChapter 9 · Powders and ceramics

Spark plasma sintering (SPS)

Pulsed current heats a pressed graphite die in minutes

Spark plasma sintering (SPS) presses powder in a graphite die while a huge pulsed electric current runs through it. The die heats itself, so the powder becomes a dense solid in minutes rather than hours. Labs use it to make new materials, such as ultra-hard ceramics, that ordinary furnaces struggle to densify.

Photo: Raihan.Rumman, CC BY-SA 3.0, via Wikimedia Commons

Spark plasma sintering · press column, current loop, densificationThe press column is drawn cut in half; the panel on the right shows what happens inside the die and what comes out. Circled numbers match the steps.

How it works

Fill the die. Powder is poured into a graphite die (a thick tube) between two graphite punches. The die is usually lined with graphite foil so the part releases.
Load and pump down. The die stack goes between the water-cooled rams of a press inside a chamber. The air is pumped out (vacuum) or replaced by argon, so the hot graphite and powder do not burn.
Press and pass current. The rams press at typically 30–100 MPa while a power supply sends pulsed direct current, thousands of amperes at a few volts, through the graphite. The die heats from inside by electrical resistance, like a toaster wire, at 100 °C per minute or more. Despite the name, there is usually no spark or plasma.
Hold briefly. After typically 5–10 minutes at peak temperature (often 1,000–2,000 °C, depending on the material), the particles bond and the pores squeeze out. The ram moves down as the powder shrinks, which shows when it is dense.
Cool and eject. With the power off, the water-cooled rams pull the heat out quickly. The dense disc is pressed out of the die, and its graphite skin is ground off before cutting or finishing.

Key facts

Materials: Ceramics (alumina, silicon carbide, borides), hard metals, tungsten and other high-melting metals, thermoelectrics, composites, very fine powders
Typical parts: Discs and simple blocks: sputtering targets, armor tiles, cutting-tool blanks, research samples
Part size: Mostly 10–50 mm discs in labs; industrial machines make parts up to a few hundred millimeters across
Precision: Rough as sintered: the diameter is set by the die, the thickness less exactly; final tolerances come from grinding
Surface finish: A gray graphite skin, ground or polished off afterward
Batch size it suits: One-offs to small batches: research, prototypes and specialty parts
Tooling and machine cost: Graphite dies are consumables costing roughly tens to hundreds of dollars; machines run from a few hundred thousand dollars (lab) to millions (industrial)
Speed: Typically 10–30 minutes per run plus cooling, one part or a small stack at a time

Strengths and limits

Strengths

Very fast: minutes instead of hours
Keeps grains (the tiny crystals in the material) very fine, which can make it stronger or even transparent
Densifies hard-to-sinter materials like borides and carbides, often without additives
Can bond layers of different materials in one run

Limits

Simple shapes only: discs, blocks, short cylinders
Small parts and low output per machine
Graphite tooling limits pressure to about 100 MPa and can add carbon to the surface
Temperature can be uneven in large parts or in powders that do not conduct

Where it's used

Research labsNew ceramics, alloys and composites; fast trials of many recipes
ElectronicsThermoelectric materials, sputtering targets for thin-film coating
Defense and aerospaceBoron carbide armor, ceramics for extreme heat
ToolsCutting-tool blanks and hard-metal parts
OpticsTransparent ceramics such as spinel windows, mostly at research scale

How to recognize it

Flat discs or short cylinders with exactly the shape of a round die.
A gray graphite skin on unfinished parts.
Ground faces on almost every finished part, since SPS parts are rarely used as they come out.

Compare with

Hot isostatic pressing9.3presses with gas from all sides and handles complex shapes, but a cycle takes hours.Powder metallurgy9.1presses cold, then sinters in a separate furnace; far cheaper per part, but less dense.Microwave sintering9.5also heats from inside for speed, but with microwaves instead of current, and without pressure.

Watch it

Prof. Neha Gupta · The principle: pulsed current through a graphite die under pressure

Haoyuevac · A real lab run, with the die glowing as it heats

ThermalTechnology1 · An industrial system: chamber, press column and controls

Taylor Sparks · What SPS is used for in industry, and its limits

See real photos

Wikipedia: Spark plasma sintering — other names (FAST, PECS), heating rates and materials; mostly text
Wikimedia Commons photo search

Remember

SPS heats the die and powder directly with a large pulsed current while pressing.
Minutes instead of hours keeps grains fine and makes hard materials dense.
It is a lab and specialty process for simple discs and blocks, not mass production.