Silicon Nitride: A Quick Guide

November 27, 2024

What is Silicon Nitride

Silicon nitride (Si₃N₄) is a polycrystalline inorganic nonmetallic chemical compound based on silicon and nitrogen synthesis, a significant important ceramic material.

Silicon nitride ceramic offers the mechanical, thermal, electrical, and chemical properties of almost all Advanced Ceramics in one, especially its ultra-high thermal shock resistance and thermal shock, as well as extremely hard and wear resistance, making it a functional and structural ceramic in many harsh environments and demanding high-tech industries have a vast range of applications.

Material Advantages

※ Unbeatable high thermal shock resistance and impact

※ Superb-high service temperature to 1300℃ in the air

※ Superior hardness and wear resistance with low-friction

※ High mechanical strength stability under high temperature

※ High flexural strengths and high fracture toughness

※ High mechanical fatigue and creep resistance

※ Low thermal expansivity similar to Si chips

※ Good electrical insulation and dielectric strength

※ High stiffness and high rigidity

※ Low density with lightweight than most metals

※ Excellent chemical stability, corrosion, and erosion resistance

Uses and Applications

※ Electronics: electrical insulators, power semiconductor devices, photoelectric display devices, etc.

※ Mechanical engineering: cutting tools, high-precision ball bearings, roller bearings, gear wheels, etc.

※ Medical: dental implants, joint prosthesis, spine repair, joint implantation, biosensors and diagnostic devices, ‌drug delivery systems, microsurgical tools, implantable medical devices, etc. ‌

※ High-temp materials: furnace ignitor, heater tubes, nozzle liners, TIG welding nozzles, Ceramic Crucible, etc.

※ Automobile industry: engine parts, turbocharger, braking system, emissions control system, etc.

※ Aerospace: turbine blades, ceramic coating, ceramic thin film, aviation instrumentation, ‌aero engine lining, etc.

※ Other fields include solar cells, valves, seal faces, ceramic wafers, heat dissipation Ceramic Substrates, weld positioning pins, silicon nitride waveguides, silicon nitride membranes, etc.

Silicon Nitride Synthesis

Silicon nitride mainly includes the following synthesis methods in the modern industry:

※ Direct Nitriding Method

High-purity silica powder is placed in a nitrogen atmosphere, and the chemical reaction is carried out at 1300 °C ~ 1400 °C to obtain silicon nitride powder. Its chemical formula is 3 Si + 2N₂→ Si₃N₄.

※ Chemical Deposition Method (CVD)

This method's primary raw materials are silicon tetrachloride, pure nitrogen, and hydrogen, which are mixed at 1000 °C ~ to 1200 °C. The purity of nitriding obtained is high. Its (Silicon Nitride CVD) chemical formula is 3SiCl₄ + 2N₂ + 6H₂ = Si₃N₄ + 12HCl

※ Si(NH2)4 Thermal Decomposition Method

silicon tetrachloride and ammonia are first combined to form Si(NH2)4 and HCL, and then Si(NH2)4 is pyrolyzed to obtain silicon nitride powder. Its chemical formula is SiCl₄ + 4NH₃ → Si(NH₂)₄ + 4HCl, 3Si(NH₂)₄ （Heating） →Si₃N₄ + 8NH₃

※ Carbonthermal Reduction Method

It is a commonly used method for preparing silicon nitride powder; the basic principle is to use carbon to reduce silicon dioxide powder in a high-temperature nitrogen environment to prepare silicon nitride powder. Its chemical formula is: 3SiO₂(s) + 6C(s) + 2N₂(g) = Si₃N₄(s) + 6CO(g)

※ Sol-gel Method

This is an advanced process for the production of silicon nitride powder. The sol-gel method uses a highly active silicon source as a precursor, which is mixed in the liquid phase to form a sol. Then, nano-scale silicon nitride powder is prepared by drying and sintering. This production method results in uniform, high-quality silicon nitride powder.

※ Self-propagation Method

This synthesis method ignites the powder body evenly mixed with silicon powder and silicon nitride through an external heating source. It uses the heat released by their reaction to synthesize further.

The above synthesis methods have advantages and disadvantages, and in practical applications, the final choice must be based on the specific requirements of product performance and cost.

Si3N4 Ceramic Crystal Structure

Silicon nitride ceramics mainly include α-Si₃N₄ (alpha silicon nitride), β-Si₃N₄ (beta silicon nitride), and γ-Si₃N₄ (cubic silicon nitride)， refer to the following Figure 1. Alpha Silicon Nitride and Beta Silicon Nitride are the most common.

Silicon Nitride Crystal Structures

With the increase of temperature, the crystalline phase of α-Si₃N₄ is converted to β-Si₃N₄ at 1400°C~1800°C, but this metamorphosis is irreversible. Therefore, the occurrence of phase transformation is beneficial to the occurrence of α-Si₃N₄ in the process of high-temperature use. In comparison, β-Si₃N₄ is a crystalline phase stable ceramic material in thermodynamics.

Si3N4 Ceramic Preparation Methods

According to different sintering methods, it can be divided into gas-pressure sintered silicon nitride,

reaction-bonded sintering silicon nitride, pressureless sintering silicon nitride, and hot pressed sintered silicon nitride. Different types of sintered silicon nitride ceramics have different grain morphology, intergranular morphology, porosity, and pore morphology, so their properties are very different.

※ Reaction-bonded Sintering Silicon Nitride

The silicon nitride powder is first molded into a green billet that is consistent with the shape of the finished product and then pre-fired in a nitrogen atmosphere furnace. The pre-fired green billet has a specific strength, which allows it to be machined. Because the shrinkage of silicon nitride material is minimal (<0.11%), the machined blank is completely sintered to obtain a product with complex structures and relatively precise size. Reaction sintering is the most commonly used method for preparing silicon nitride ceramics.

※ Pressureless Sintering Silicon Nitride

The pressureless sintering method is carried out in a nitrogen atmosphere of 1700°C~1800°C at atmospheric pressure. The dense silicon nitride ceramics are prepared by using the decomposition

reaction of silicon nitride powder at high temperatures. The silicon nitride ceramics prepared by this method have high mechanical strength.

※ Gas Pressure Sintering Silicon Nitride

Pneumatic sintering is generally carried out at about 2000°C, 1~10MPa. Silicon nitride powder will be added to the high-temperature sintering additives such as MgO and Y2O3 to promote the grain growth of silicon nitride, and silicon nitride ceramic products with purity greater than 99% and high toughness can be obtained.

※ Hot Pressed Sintering Silicon Nitride

The hot-pressing sintering method involves adding a small amount of MgO, Al₂O₃, and high-purity silicon nitride powder above 1600 °C and 1916MPa. MgF₂, Fe₂O₃, and other sintering additives are sintered to obtain high strength, high hardness, and high-density silicon nitride ceramics.

Silicon Nitride Materials Ceramic Properties

Mechanical Properties

Property	Unit	Value
Property	Unit	Gas Pressure Sintering Si₃N₄	Reaction-bonded Sintering Si₃N₄	Pressureless Sintering Si₃N₄	Hot Pressed Si₃N₄
Density	g/cm³	≧3.2	≧2.7	≧3.0	≧3.25
Hardness	HRA	92~93	82~85	91~92	92~93
Flexural Strength with 4 Points @ 25℃	MPa	＞900	250~350	700~800	＞1000
Poisson Ratio	-	0.25	0.24	0.24	0.28
Elastic Modulus	GPa	300~320	160~200	290~320	310
Compressive Strength	MPa	≧3200	≧1500	≧2500	≧3900
Fracture Toughness	MPa m^1/2	5~7	3~4	5~6	≧7
Coefficient of Friction	-	≈0.1	≈0.1	≈0.2~0.3	≈0.1

Thermal Properties

Property	Unit	Value
Property	Unit	Gas Pressure Sintering Si₃N₄	Reaction-bonded Sintering Si₃N₄	Pressureless Sintering Si₃N₄	Hot Pressed Si₃N₄
Specific Heat	J/g·K	0.70	0.50	-	0.71
Thermal Conductivity	W/m.K	30~33	18.5	-	23
Thermal Expansion Coefficient (CTE)	1*10^-6/℃	2.3~3.2	2.2~2.9	2.5~3.1	3.5
Thermal Shock Resistance	℃	800	450	25	1000

Electrical Properties

Property	Unit	Value
Property	Unit	Gas pressure Sintering Si₃N₄	Reaction-bonded Sintering Si₃N₄	Pressureless Sintering Si₃N₄	Hot Pressed Si₃N₄
Volume Resistivity @ 25℃	Ohm. cm	1*10¹⁴	1*10¹⁵	1*10¹⁴	1*10¹⁴
Dielectric Constant @ 25℃	MHz	9.4~9.5	9.4~9.5	7~9	8.3
Breakdown Voltage	Ac-KV/mm	15	15	10~15	40
Loss Tangent @ 25℃	MHz	0.001~0.10	0.001~0.10	0.001~0.10	0.001~0.10

Optical Properties

Property	Unit	Value
Property	Unit	Gas pressure Sintering Si₃N₄	Reaction-bonded Sintering Si₃N₄	Pressureless Sintering Si₃N₄	Hot Pressed Si₃N₄
Absorption Coefficient @ 0.1~1.1 THz	Cm^-1	＜0.01	＜0.01	＜0.01	＜0.01
Refractive Index @ 632.8nm	-	1.9~2.2	2.8	2.8	2.8
Transmission Spectrum	nm	200~1100	100~800	-	1000~1100

Final Thoughts

With the further exploration and research of the mechanism and properties of silicon nitride ceramic materials, especially the improvement of silicon nitride powder preparation technology and the advent of large-scale equipment, It is foreseeable that silicon nitride ceramics will play a more critical role in various demanding industries and have a more comprehensive range of applications.

Author:

Mr. Andy Chen

E-mail:

andy@jinghui0738.com

Phone/WhatsApp:

+86 13760126904