Silicon nitride fiber monolithic ceramic and preparation method thereof
By preparing silicon nitride fiber monolithic ceramics, and utilizing a mixed powder of silicon nitride, alumina, ytterbium oxide and yttrium oxide, and a boron nitride interface layer, combined with wet spinning and hot pressing sintering processes, the problem of insufficient laser ablation resistance of silicon nitride ceramics was solved, and high-strength, ablation-resistant silicon nitride fiber ceramic materials were realized.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-16
- Publication Date
- 2026-04-14
AI Technical Summary
Existing silicon nitride ceramics have shortcomings in terms of resistance to laser ablation, and the complexity of their material structure and composition makes it difficult to control, which affects their reliability and safety in applications such as aerospace.
A silicon nitride fiber monolithic ceramic was prepared by using a mixed powder with silicon nitride, aluminum oxide, ytterbium oxide and yttrium oxide as the main components and wet spinning technology to prepare a silicon nitride fiber precursor. An interface layer of boron nitride and aluminum oxide was formed on its surface. Combined with hot pressing sintering process, silicon nitride fiber monolithic ceramic was prepared.
It improves the flexural strength and fracture toughness of silicon nitride fiber ceramics, while significantly enhancing their resistance to laser ablation. The process is simple, low-cost, shortens the preparation time, and the material surface is smooth and easy to process.
Smart Images
Figure CN121850683A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of ceramics and their manufacturing methods, specifically a silicon nitride fiber monolithic ceramic and its preparation method. Background Technology
[0002] Silicon nitride ceramics are an important structural ceramic material with excellent properties such as high thermal stability, strong oxidation resistance, high hardness, wear resistance, high temperature resistance, and high dimensional accuracy. They are widely used in aerospace, chemical, machining, and biomedicine fields. With the rapid development of aerospace in recent years, silicon nitride ceramics have attracted the attention of many researchers and have become one of the most promising materials for research.
[0003] Over the past two decades, with the continuous increase in aircraft speed and the increasing complexity of service environments, coupled with the upgrading of aircraft interception methods (laser weapons) by other countries, higher demands have been placed on the performance of silicon nitride ceramics, especially their ablation resistance. This remains one of the challenges to be solved. Therefore, to ensure the good reliability and safety of silicon nitride materials during use, improving their laser ablation resistance without reducing their mechanical properties has become urgent. Further in-depth research into ceramic materials by the materials science community has revealed that, according to current research approaches, achieving higher performance necessitates increasingly complex and difficult-to-control material structures and compositions. Summary of the Invention
[0004] Purpose of the invention: In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a method for preparing silicon nitride fiber monolithic ceramics that is simple to process, easy to operate, low in cost, highly designable, and has excellent processability. Another purpose of this invention is to provide a silicon nitride fiber monolithic ceramic with high flexural strength, good fracture toughness, and good ablation resistance.
[0005] Technical solution: The present invention provides a method for preparing silicon nitride fiber monolithic ceramics, comprising the following steps:
[0006] Step 1: Silicon nitride, aluminum oxide, ytterbium oxide, and yttrium oxide are mixed with a solvent, and after ball milling, drying, grinding, and sieving, a main material mixed powder is obtained; the dispersant, binder, plasticizer, and solvent are stirred until completely mixed to obtain a mixed liquid;
[0007] Step 2: The mixture is ball-milled together with the main material powder to prepare the spinning solution. Then the spinning solution is evenly sprayed into the coagulation bath through the spinning nozzle. After coagulation, the silicon nitride fiber precursor is obtained.
[0008] Step 3: The silicon nitride fiber precursor is immersed in an interface layer slurry obtained by mixing boron nitride and alumina powder using the dip-coating method to modify it, thereby obtaining a silicon nitride fiber precursor with an interface layer.
[0009] Step 4: Cut the dried silicon nitride fiber precursor with interface layer according to the size of the pre-compression mold, arrange it uniaxially or crosswise in the mold, and pre-compress it to obtain the fiber monolith pre-compression blank.
[0010] Step 5: Dry the fiber monolithic pre-pressed preform until the quality of the fiber monolithic preform no longer changes, and obtain the preform;
[0011] Step 6: Remove the glue from the blank;
[0012] Step 7: Hot-press sinter the material obtained in Step 6 to obtain silicon nitride fiber monolithic ceramic.
[0013] Furthermore, in step one, the mass ratio of silicon nitride, aluminum oxide, ytterbium oxide, and yttrium oxide is 90~100:3~5:1~3:7~12, the particle size of the raw material before silicon nitride ball milling is 0.5~2 μm, the ball milling speed is 450~650 r / min, and the ball milling time is 4~24 h.
[0014] Further, in step one, the binder is polyethylene glycol butyral, the dispersant is polyethylene glycol, the plasticizer is glycerol, the solvent is anhydrous ethanol, and the mass ratio of binder, dispersant, plasticizer and solvent is 10~20:10:3~5:70~75.
[0015] Furthermore, in step two, the coagulation bath is water, the temperature of the coagulation bath is 0~10℃, and the diameter of the obtained silicon nitride fiber precursor is 200~800 μm.
[0016] Furthermore, in step three, the mass ratio of boron nitride to alumina powder is 1~1.5:3~4, and the particle size is 1~5 μm.
[0017] Furthermore, in step three, the soaking time is 10~30s, and the thickness of the interface layer is 5~20 μm. The thickness of the interface layer can be controlled by the soaking time.
[0018] Furthermore, in step four, the pre-compression pressure is 10~15MPa, and the pre-compression holding time is 10~20min.
[0019] Furthermore, in step six, the adhesive removal conditions are as follows: the temperature is increased from room temperature to 600-700 ℃ at a heating rate of 0.5-5.0 ℃ / min, and held at that temperature for 1-2 h.
[0020] Furthermore, in step seven, the conditions for pressure sintering heat are as follows: heating from room temperature to 1780~1850 ℃ at a heating rate of 10~15 ℃ / min, holding at temperature and pressure for 1~2 h, and then cooling with the furnace.
[0021] The silicon nitride fiber monolithic ceramic of this invention has a flexural strength of 550~630 MPa and a fracture toughness of 10~15 MPa·m. 1 / 2 Under 1100W laser ablation, the ablation rate was 4.7 × 10⁻⁶. −6 For speeds below m / s, the mass ablation rate is 6.7 × 10⁻⁶. −4 Below g / s.
[0022] Preparation principle: Silicon nitride is used as the main material, and alumina, ytterbium oxide, and yttrium oxide are used as sintering aids. Binders, dispersants, plasticizers, and solvents are used to prepare the slurry. A coagulation bath is used to obtain the silicon nitride fiber precursor. The silicon nitride fiber precursor is pressed into a green body, which is then dried, debinded, and sintered to obtain silicon nitride fiber monolithic ceramic.
[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant features:
[0024] 1. The process is simple and low-cost, achieving the improvement of laser ablation resistance without compromising the toughness of ceramic materials; it greatly shortens the fiber preparation time, and the coating is uniformly applied to the surface of the fiber precursor. During the laser ablation process, various glass phases formed delay or even prevent the interior from being ablated by the laser, thus improving the laser ablation resistance of the ceramic materials. The final silicon nitride fiber monolithic ceramic material improves the laser ablation resistance of the material without reducing its mechanical strength.
[0025] 2. The formulation of silicon nitride and sintering aid was optimized, and the silicon nitride and sintering aid were mechanically activated. Based on further thorough mixing, the performance was improved and its ablation resistance was enhanced.
[0026] 3. The wet spinning technique for preparing silicon nitride fiber precursors is simple, easy to operate, and low in cost, significantly shortening the fiber preparation time and improving preparation efficiency. The prepared silicon nitride fibers have a smooth and continuous surface, are easy to process and deform, have a diameter of 200~800 μm, and possess a certain degree of toughness, allowing the fibers to be cut into various shapes, thus overcoming the drawback of poor fiber flowability in existing preparations.
[0027] 4. A certain amount of glycerol was added during the preparation of the spinning solution, which increased the fluidity of the spinning solution. When the spinning solution enters the spinning nozzle for solidification, it will not cause blockage. Moreover, the resulting fibers have better toughness and are easier to process.
[0028] 5. The coagulation bath is water, and the temperature of the coagulation bath is room temperature, which greatly reduces costs. Attached Figure Description
[0029] Figure 1 This is a physical image of the product obtained in Embodiment 1 of the present invention;
[0030] Figure 2 This is a physical image of the product obtained in Embodiment 1 of the present invention;
[0031] Figure 3 These are microstructure diagrams of the Si3N4 / BN fiber monolithic ceramic material of the present invention, wherein (a) is Example 1 and (b) is Example 3;
[0032] Figure 4 This is a SEM image of the surface of the material obtained in Example 2 of this invention after laser ablation;
[0033] Figure 5 This is an EDS analysis result diagram of the material obtained in Example 2 of the present invention after laser ablation. Detailed Implementation
[0034] Unless otherwise specified, all materials and reagents used in the following embodiments are commercially available. Experimental methods not specifically described in the embodiments are generally performed under standard conditions or as recommended by the manufacturer.
[0035] Example 1
[0036] A method for preparing silicon nitride fiber monolithic ceramics includes the following steps:
[0037] (1) Mechanically activated high-energy milled silicon nitride powder: First, 90g of silicon nitride, 3.6g of aluminum oxide and 6.4g of yttrium oxide were mixed with anhydrous ethanol for wet mixing. The slurry was ball-milled for 48 h at a speed of 450 r / min. Then, the powder was dried at 95℃ for 48 h using a rotary evaporator. The dried powder was then ground and sieved to obtain fine and uniform mixed powder.
[0038] (2) Preparation of mixed adhesive: Prepare a mixed solution of 10g polyethylene glycol butyral, 10g polyethylene glycol, 5g glycerol and 75g anhydrous ethanol, and mechanically stir for 6 h to fully dissolve the solution.
[0039] (3) Preparation of silicon nitride fiber monolithic precursor: After dissolving, add the mixed powder, ball mill for 48 h to obtain spinning solution, use a spinning nozzle to spray the spinning solution into the coagulation bath, soak in the coagulation bath for 20 min to obtain silicon nitride fiber precursor, the coagulation bath is deionized water, the temperature is room temperature, the diameter of the silicon nitride fiber precursor is 550 μm.
[0040] (4) Silicon nitride interface modification: The silicon nitride fiber precursor was modified by impregnation. 3.75g of boron nitride was mixed in deionized water and mechanically stirred for 3 h. Then 1.25g of alumina powder was added and stirred for another 6 h to obtain a uniformly mixed interface layer slurry. Boron nitride and alumina were mixed at a mass ratio of 1:3. The silicon nitride fiber monolithic precursor was immersed in the interface layer slurry for 10s. The interface layer thickness of the fiber monolithic precursor was changed by changing the immersion time. The BN interface layer thickness was 10.14 μm.
[0041] (5) Pre-compression molding: First, the coated fiber monolith precursor is dried in an oven at 60°C. The impregnated silicon nitride fiber precursor is cut into fibers of uniform length according to the size of the pre-compression mold. The fibers are arranged uniaxially or crosswise in the mold and pre-compressed at room temperature using a hydraulic press. The pressure is maintained at 10 MPa for 20 min to obtain the fiber monolith pre-compressed blank.
[0042] (6) Drying of the preform: The fiber monolith pre-pressed preform is placed indoors for natural drying for 14 days until the quality of the fiber monolith preform no longer changes. The drying of the preform is then complete.
[0043] (7) Removing the glue from the green body: The dried green body is placed in a box-type resistance furnace for removing the glue. The glue removal temperature is 600 ℃, the holding time is 1 h, and the heating rate is 1 ℃ / min.
[0044] (8) Hot pressing sintering: The debinding green body is hot-pressed in a hot pressing sintering furnace, with the temperature increased to 1780 ℃ at a heating rate of 10 ℃ / min, held for 1 h, and the pressure at 30 MPa. After cooling in the furnace, silicon nitride fiber monolithic ceramic material is obtained, with a flexural strength of 587.17 MPa and a fracture toughness of 11.76 MPa·m. 1 / 2 In 1100W laser ablation, the linear ablation rate was 4.5 × 10⁻⁶. −6 m / s, mass ablation rate of 6.5 × 10 −4 g / s.
[0045] Example 2
[0046] A method for preparing silicon nitride fiber monolithic ceramics includes the following steps:
[0047] (1) Mechanically activated high-energy milled silicon nitride powder: First, 100g of silicon nitride, 6g of aluminum oxide and 12g of yttrium oxide were mixed with anhydrous ethanol for wet mixing. The slurry was then ball-milled for 24 hours at a speed of 650r / min. Subsequently, the powder was dried at 92℃ for 60 hours using a rotary evaporator. The dried powder was then ground and sieved to obtain fine and uniform mixed powder.
[0048] (2) Preparation of mixed adhesive: Prepare a mixed solution of 15g polyethylene glycol butyral, 10g polyethylene glycol, 3g glycerol and 70g anhydrous ethanol, and mechanically stir for 10 h to fully dissolve the solution.
[0049] (3) Preparation of silicon nitride fiber monolithic precursor: The mixed powder and solution were added and ball-milled for 48 h to obtain spinning solution. The solution was then introduced into the coagulation bath through the spinning nozzle. The spinning solution was sprayed into the coagulation bath through the spinning nozzle. After soaking in the coagulation bath for 20 min, the silicon nitride fiber precursor was obtained. The coagulation bath was deionized water and the temperature was room temperature. The diameter of the silicon nitride fiber precursor was 650 μm.
[0050] (4) Silicon nitride interface modification: The silicon nitride fiber precursor was modified by impregnation. 5.25g of boron nitride was mixed in deionized water and mechanically stirred for 2h. Then 1.75g of alumina powder was added and stirred for another 8h to obtain a uniformly mixed interface layer slurry. Boron nitride and alumina were mixed at a mass ratio of 1:3. The silicon nitride fiber monolithic precursor was immersed in the interface layer slurry for 15min. The interface layer thickness of the fiber monolithic precursor was changed by changing the immersion time. The thickness of the BN interface layer was 12.57 μm.
[0051] (5) Pre-compression molding: First, the coated fiber monolith precursor is dried in an oven at 60°C. The impregnated silicon nitride fiber precursor is cut into fibers of uniform length according to the size of the pre-compression mold. The fibers are arranged in a single axis or cross in the mold. The pre-compression is performed at room temperature using a hydraulic press and held at 15MPa for 20 minutes to obtain the fiber monolith pre-compression blank.
[0052] (6) Drying of the preform: The fiber monolith pre-pressed preform is placed indoors for natural drying for 14 days until the quality of the fiber monolith preform no longer changes. The drying of the preform is then complete.
[0053] (7) Removing the glue from the green body: The dried green body is placed in a box-type resistance furnace for removing the glue. The removal temperature is 600℃, the holding time is 1 h, and the heating rate is 2.0℃ / min.
[0054] (8) Hot-press sintering: The debinding blank is hot-pressed in a hot-press sintering furnace, with the temperature increased to 1800 ℃ at a heating rate of 20 ℃ / min, held for 1 h, and pressure of 30 MPa. After furnace cooling, silicon nitride fiber monolithic ceramic material is obtained, with a flexural strength of 609.32 MPa and a fracture toughness of 13.93 MPa. 1 / 2 In 1100W laser ablation, the linear ablation rate was 3.8 × 10⁻⁶. −6m / s, mass ablation rate of 6.7 × 10 −4 g / s.
[0055] Example 3
[0056] A method for preparing silicon nitride fiber monolithic ceramics includes the following steps:
[0057] (1) Mechanically activated high-energy milled silicon nitride powder: 90g silicon nitride, 3g alumina and 7g yttrium oxide were mixed with anhydrous ethanol and wet-mixed. The slurry was ball-milled for 48 h at a speed of 550 r / min. Then, it was dried at 93℃ for 24 h using a rotary evaporator. The dried powder was then ground and sieved to obtain fine and uniform mixed powder.
[0058] (2) Preparation of mixed adhesive: 20g polyethylene glycol butyral, 10g polyethylene glycol, 5g glycerol and 75g anhydrous ethanol are mixed and mechanically stirred for 6 h to fully dissolve the solution.
[0059] (3) Preparation of silicon nitride fiber monolithic precursor: After dissolving, add the mixed powder, ball mill for 48 h to obtain spinning solution, use a spinning nozzle to spray the spinning solution into a coagulation bath, soak in the coagulation bath for 20 min to obtain silicon nitride fiber precursor, the coagulation bath is deionized water, the temperature is room temperature; wherein, the diameter of silicon nitride fiber precursor is 700 μm.
[0060] (4) Silicon nitride interface modification: The silicon nitride fiber precursor was modified by impregnation. 6.75g of boron nitride was mixed in deionized water and mechanically stirred for 4 h. Then 2.25g of alumina powder was added and stirred for another 8 h to obtain a uniformly mixed interface layer slurry. Boron nitride and alumina were mixed at a mass ratio of 1:3. The silicon nitride fiber monolithic precursor was immersed in the interface layer slurry for 20 min. The interface layer thickness of the fiber monolithic precursor was changed by changing the immersion time. The BN interface layer thickness was 16.32 μm.
[0061] (5) Pre-compression molding: First, the coated fiber monolith precursor is dried in an oven at 70°C. The impregnated silicon nitride fiber precursor is cut into fibers of uniform length according to the size of the pre-compression mold. The fibers are arranged in a single axis or cross in the mold. The pre-compression is performed at room temperature using a hydraulic press and held at 15 MPa for 10 min to obtain the fiber monolith pre-compression blank.
[0062] (6) Drying of the preform: The fiber monolith pre-pressed preform is placed indoors for natural drying for 14 days until the quality of the fiber monolith preform no longer changes. The drying of the preform is then complete.
[0063] (7) Removing the glue from the green body: The dried green body is placed in a box-type resistance furnace for removing the glue. The glue removal temperature is 600 ℃, the holding time is 1 h, and the heating rate is 5.0 ℃ / min.
[0064] (8) Hot-press sintering: The debinding blank is hot-pressed in a hot-press sintering furnace, with the temperature increased to 1800 ℃ at a heating rate of 10 ℃ / min, held for 2 h, and pressure of 30 MPa. After furnace cooling, silicon nitride fiber monolithic ceramic material is obtained, with a flexural strength of 550.39 MPa and a fracture toughness of 14.65 MPa. 1 / 2 In 1100W laser ablation, the linear ablation rate was 3.5 × 10⁻⁶. −6 m / s, mass ablation rate of 5.5 × 10 −4 g / s.
[0065] Table 1 Performance parameters of silicon nitride fiber monolithic ceramic structures in embodiments 1, 2, and 3
[0066]
[0067] As shown in Table 1 above, the higher the sintering temperature, the better the mechanical properties of the prepared ceramics. When the sintering temperature is 1800 ℃ and the holding time is 1.5 h, the prepared silicon nitride / boron nitride fiber monolithic ceramics exhibit excellent flexural strength and fracture toughness, but the ablation resistance is not the best. When the thickness of the BN interface layer is thicker, the fracture toughness increases, but the flexural strength decreases, while the ablation resistance increases.
[0068] Figure 1 , Figure 2 The images show actual photos of the silicon nitride fiber precursors prepared by methods 1 and 2 of the present invention, respectively. It can be seen that the silicon nitride fiber precursors are dense, continuous, and have a smooth surface, exhibiting a good fiber appearance.
[0069] Figure 3 (a) and Figure 3 (b) The microstructures of Si3N4 / BN fiber monolithic ceramic materials in embodiments 1 and 3 of the present invention are shown respectively. It can be clearly seen from the figure that the Si3N4 / BN fiber monolithic ceramic materials obtained after pre-compression molding, debinding and sintering of different silicon nitride fiber precursors are Si3N4 / BN fiber monolithic ceramic materials. The Si3N4 cells have a rhombic structure. The boundary between the rhombic cells is the boron nitride interface. The larger diameter of the silicon nitride fiber precursor will result in a larger Si3N4 cell.
[0070] like Figures 4-5The BN interface layer surrounding the Si3N4 fiber cells begins to oxidize above 1000℃, forming a B2O3 liquid phase. As oxidation progresses, the B2O3 liquid phase vaporizes. The Si3N4 fiber cells are oxidized above 1200℃, and the resulting Y2Si2O7 and SiO2 crystals are surrounded by a glassy phase. When the oxidation temperature rises to 1400℃, the Si3N4 fiber cells are severely damaged, resulting in an oxide layer composed of large-sized Y2Si2O7 crystals and a glassy phase. As ablation continues, the glassy oxide layer provides protection, enhancing its ablation resistance.
Claims
1. A method for preparing silicon nitride fiber monolithic ceramic, characterized in that, Includes the following steps: Step 1: Silicon nitride, aluminum oxide, ytterbium oxide, and yttrium oxide are mixed with a solvent, and after ball milling, drying, grinding, and sieving, a main material mixed powder is obtained; the dispersant, binder, plasticizer, and solvent are stirred until completely mixed to obtain a mixed liquid; Step 2: The mixture is ball-milled together with the main material powder to prepare the spinning solution. Then the spinning solution is evenly sprayed into the coagulation bath through the spinning nozzle. After coagulation, the silicon nitride fiber precursor is obtained. Step 3: The silicon nitride fiber precursor is immersed in an interface layer slurry obtained by mixing boron nitride and alumina powder using the dip-coating method to modify it, thereby obtaining a silicon nitride fiber precursor with an interface layer. Step 4: Cut the dried silicon nitride fiber precursor with interface layer according to the size of the pre-compression mold, arrange it uniaxially or crosswise in the mold, and pre-compress it to obtain the fiber monolith pre-compression blank. Step 5: Dry the fiber monolithic pre-pressed preform until the quality of the fiber monolithic preform no longer changes, and obtain the preform; Step 6: Remove the glue from the blank; Step 7: Hot-press sinter the material obtained in Step 6 to obtain silicon nitride fiber monolithic ceramic.
2. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step one, the mass ratio of silicon nitride, aluminum oxide, ytterbium oxide and yttrium oxide is 90~100:3~5:1~3:7~12, the particle size of the raw material before ball milling of silicon nitride is 0.5~2 μm, the ball milling speed is 450~650 r / min, and the ball milling time is 4~24 h.
3. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step one, the binder is polyethylene glycol butyral, the dispersant is polyethylene glycol, the plasticizer is glycerol, and the solvent is anhydrous ethanol. The mass ratio of the binder, dispersant, plasticizer, and solvent is 10~20:10:3~5:70~75.
4. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step two, the coagulation bath is water, the temperature of the coagulation bath is 0~10℃, and the diameter of the obtained silicon nitride fiber precursor is 200~800 μm.
5. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step three, the mass ratio of boron nitride to aluminum oxide powder is 1~1.5:3~4, and the particle size is 1~5 μm.
6. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step three, the soaking time is 10-30 seconds and the thickness of the interface layer is 5-20 μm. The thickness of the interface layer can be controlled by the soaking time.
7. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step four, the pre-compression pressure is 10~15MPa, and the pre-compression holding time is 10~20min.
8. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step six, the adhesive removal conditions are as follows: the temperature is increased from room temperature to 600-700 ℃ at a heating rate of 0.5-5.0 ℃ / min, and held at that temperature for 1-2 h.
9. The method for preparing silicon nitride fiber monolithic ceramic according to claim 1, characterized in that: In step seven, the conditions for pressure sintering heat are as follows: heating from room temperature to 1780-1850 ℃ at a heating rate of 10-15 ℃ / min, holding at temperature and pressure for 1-2 h, and then cooling with the furnace.
10. The silicon nitride fiber monolithic ceramic obtained by the preparation method of silicon nitride fiber monolithic ceramic according to any one of claims 1 to 9, characterized in that: The flexural strength is 550~630 MPa, and the fracture toughness is 10~15 MPa·m. 1 / 2 Under 1100W laser ablation, the ablation rate was 4.7 × 10⁻⁶. −6 For speeds below m / s, the mass ablation rate is 6.7 × 10⁻⁶. −4 Below g / s.